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This also adds the `hasattr` and `getattr` functions which are needed
with the new attribute behavior. The new functions behave exactly like
their Python counterparts.

Similarly `object` gets a `contains` method which calls `__contains__`,
i.e. it's the same as the `in` keyword in Python.

Add unit tests and documentation for the chrono cast.

Take load_type by nested type_caster template arguments instead of by
full type_caster type.

This adds a static local variable (in dead code unless actually needed)
in the overload code that is used for storage if the overload is for
some convert-by-value type (such as numeric values or std::string).

This has limitations (as written up in the advanced doc), but is better
than simply not being able to overload reference or pointer methods.

This clears the Python error at the error_already_set throw site, thus
allowing Python calls to be made in destructors which are triggered by
the exception. This is preferable to the alternative, which would be
guarding every Python API call with an error_scope.

This effectively flips the behavior of error_already_set. Previously,
it was assumed that the error stays in Python, so handling the exception
in C++ would require explicitly calling PyErr_Clear(), but nothing was
needed to propagate the error to Python. With this change, handling the
error in C++ does not require a PyErr_Clear() call, but propagating the
error to Python requires an explicit error_already_set::restore().

The change does not break old code which explicitly calls PyErr_Clear()
for cleanup, which should be the majority of user code. The need for an
explicit restore() call does break old code, but this should be mostly
confined to the library and not user code.

The previous commit to address #392 triggers a compiler warning about
returning a reference to a local variable, which is *not* a false alarm:
the following:

    py::cast<int &>(o)

(which happens internally in an overload declaration) really is
returning a reference to a local, because the cast operators for the
type_caster for numeric types returns a reference to its own member.

This commit adds a static_assert to make that a compilation failure
rather than returning a reference into about-to-be-freed memory.

Incidentally, this is also a fix for #219, which is exactly the same
issue: we can't reference numeric primitives that are cast from
wrappers around python numeric types.

Need to use the intrinsic type, not the raw type.

Fixes #392.

This allows a slightly cleaner base type specification of:

    py::class_<Type, Base>("Type")

as an alternative to

    py::class_<Type>("Type", py::base<Base>())

As with the other template parameters, the order relative to the holder
or trampoline types doesn't matter.

This also includes a compile-time assertion failure if attempting to
specify more than one base class (but is easily extendible to support
multiple inheritance, someday, by updating the class_selector::set_bases
function to set multiple bases).

The current pybind11::class_<Type, Holder, Trampoline> fixed template
ordering results in a requirement to repeat the Holder with its default
value (std::unique_ptr<Type>) argument, which is a little bit annoying:
it needs to be specified not because we want to override the default,
but rather because we need to specify the third argument.

This commit removes this limitation by making the class_ template take
the type name plus a parameter pack of options.  It then extracts the
first valid holder type and the first subclass type for holder_type and
trampoline type_alias, respectively.  (If unfound, both fall back to
their current defaults, `std::unique_ptr<type>` and `type`,
respectively).  If any unmatched template arguments are provided, a
static assertion fails.

What this means is that you can specify or omit the arguments in any
order:

    py::class_<A, PyA> c1(m, "A");
    py::class_<B, PyB, std::shared_ptr<B>> c2(m, "B");
    py::class_<C, std::shared_ptr<C>, PyB> c3(m, "C");

It also allows future class attributes (such as base types in the next
commit) to be passed as class template types rather than needing to use
a py::base<> wrapper.

With this change arg_t is no longer a template, but it must remain so
for backward compatibility. Thus, a non-template arg_v is introduced,
while a dummy template alias arg_t is there to keep old code from
breaking. This can be remove in the next major release.

The implementation of arg_v also needed to be placed a little earlier in
the headers because it's not a template any more and unpacking_collector
needs more than a forward declaration.

MSVC fails to compile if the constructor is defined out-of-line.
The error states that it cannot deduce the type of the default template
parameter which is used for SFINAE.

The variadic handle::operator() offers the same functionality as well
as mixed positional, keyword, * and ** arguments. The tests are also
superseded by the ones in `test_callbacks`.

A Python function can be called with the syntax:
```python
foo(a1, a2, *args, ka=1, kb=2, **kwargs)
```
This commit adds support for the equivalent syntax in C++:
```c++
foo(a1, a2, *args, "ka"_a=1, "kb"_a=2, **kwargs)
```

In addition, generalized unpacking is implemented, as per PEP 448,
which allows calls with multiple * and ** unpacking:
```python
bar(*args1, 99, *args2, 101, **kwargs1, kz=200, **kwargs2)
```
and
```c++
bar(*args1, 99, *args2, 101, **kwargs1, "kz"_a=200, **kwargs2)
```

Currently pybind11 only supports std::unique_ptr<T> holders by default
(other holders can, of course, be declared using the macro).  PR #368
added a `py::nodelete` that is intended to be used as:

    py::class_<Type, std::unique_ptr<Type, py::nodelete>> c("Type");

but this doesn't work out of the box.  (You could add an explicit
holder type declaration, but this doesn't appear to have been the
intention of the commit).

This commit fixes it by generalizing the unique_ptr type_caster to take
both the type and deleter as template arguments, so that *any*
unique_ptr instances are now automatically handled by pybind.  It also
adds a test to test_smart_ptr, testing both that py::nodelete (now)
works, and that the object is indeed not deleted as intended.

This allows exposing a dict-like interface to python code, allowing
iteration over keys via:

    for k in custommapping:
        ...

while still allowing iteration over pairs, so that you can also
implement 'dict.items()' functionality which returns a pair iterator,
allowing:

    for k, v in custommapping.items():
        ...

example-sequences-and-iterators is updated with a custom class providing
both types of iteration.

PR #329 generates the following warning under MSVC:

    ...\cast.h(202): warning C4456: declaration of 'it' hides previous local declaration

This renames the second iterator to silence it.

reference_internal requires an `instance` field to track the returned
reference's parent, but that's just a duplication of what
keep_alive<0,1> does, so use a keep alive to do this to eliminate the
duplication.

The pointer to the first member of a class instance is the same as the
pointer to instance itself; pybind11 has some workarounds for this to
not track registered instances that have a registered parent with the
same address.  This doesn't work everywhere, however: issue #328 is a
failure of this for a mutator operator which resolves its argument to
the parent rather than the child, as is needed in #328.

This commit resolves the issue (and restores tracking of same-address
instances) by changing registered_instances from an unordered_map to an
unordered_multimap that allows duplicate instances for the same pointer
to be recorded, then resolves these differences by checking the type of
each matched instance when looking up an instance.  (A
unordered_multimap seems cleaner for this than a unordered_map<list> or
similar because, the vast majority of the time, the instance will be
unique).

Currently pybind11 always translates values returned by Python functions
invoked from C++ code by copying, even when moving is feasible--and,
more importantly, even when moving is required.

The first, and relatively minor, concern is that moving may be
considerably more efficient for some types.  The second problem,
however, is more serious: there's currently no way python code can
return a non-copyable type to C++ code.

I ran into this while trying to add a PYBIND11_OVERLOAD of a virtual
method that returns just such a type: it simply fails to compile because
this:

    overload = ...
    overload(args).template cast<ret_type>();

involves a copy: overload(args) returns an object instance, and the
invoked object::cast() loads the returned value, then returns a copy of
the loaded value.

We can, however, safely move that returned value *if* the object has the
only reference to it (i.e. if ref_count() == 1) and the object is
itself temporary (i.e. if it's an rvalue).

This commit does that by adding an rvalue-qualified object::cast()
method that allows the returned value to be move-constructed out of the
stored instance when feasible.

This basically comes down to three cases:

- For objects that are movable but not copyable, we always try the move,
  with a runtime exception raised if this would involve moving a value
  with multiple references.
- When the type is both movable and non-trivially copyable, the move
  happens only if the invoked object has a ref_count of 1, otherwise the
  object is copied.  (Trivially copyable types are excluded from this
  case because they are typically just collections of primitive types,
  which can be copied just as easily as they can be moved.)
- Non-movable and trivially copy constructible objects are simply
  copied.

This also adds examples to example-virtual-functions that shows both a
non-copyable object and a movable/copyable object in action: the former
raises an exception if returned while holding a reference, the latter
invokes a move constructor if unreferenced, or a copy constructor if
referenced.

Basically this allows code such as:

    class MyClass(Pybind11Class):
        def somemethod(self, whatever):
            mt = MovableType(whatever)
            # ...
            return mt

which allows the MovableType instance to be returned to the C++ code
via its move constructor.

Of course if you attempt to violate this by doing something like:

    self.value = MovableType(whatever)
    return self.value

you get an exception--but right now, the pybind11-side of that code
won't compile at all.