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PR #425 removed the bool operator from attribute accessors.  This is
likely in use by existing code as it was the only way before #425 added
the `hasattr` function to check for the existence of an attribute, via:

    if (obj.attr("foo")) { ... }

This commit adds it back in for attr and item accessors, but with a
deprecation warning to use `hasattr(obj, ...)` or `obj.contains(...)`
instead.

`auto var = l[0]` has a strange quirk: `var` is actually an accessor and
not an object, so any later assignment of `var = ...` would modify l[0]
instead of `var`. This is surprising compared to the non-auto assignment
`py::object var = l[0]; var = ...`.

By overloading `operator=` on lvalue/rvalue, the expected behavior is
restored even for `auto` variables.

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.

The custom exception handling added in PR #273 is robust, but is overly
complex for declaring the most common simple C++ -> Python exception
mapping that needs only to copy `what()`.  This add a simpler
`py::register_exception<CppExp>(module, "PyExp");` function that greatly
simplifies the common basic case of translation of a simple CppException
into a simple PythonException, while not removing the more advanced
capabilities of defining custom exception handlers.

Fix spaces before parens for style guide.

Allowed durations and non system clocks to be set from floats.

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.

This commit adds support for forcing alias type initialization by
defining constructors with `py::init_alias<arg1, arg2>()` instead of
`py::init<arg1, arg2>()`.  Currently py::init<> only results in Alias
initialization if the type is extended in python, or the given
arguments can't be used to construct the base type, but can be used to
construct the alias.  py::init_alias<>, in contrast, always invokes the
constructor of the alias type.

It looks like this was already the intention of
`py::detail::init_alias`, which was forward-declared in
86d825f3, but was apparently never
finished: despite the existance of a .def method accepting it, the
`detail::init_alias` class isn't actually defined anywhere.

This commit completes the feature (or possibly repurposes it), allowing
declaration of classes that will always initialize the trampoline which
is (as I argued in #397) sometimes useful.

Switch count_t to use constexpr_sum (under non-MSVC), and then make
all_of_t/any_of_t use it instead of doing the sum itself.

For MSVC, count_t is still done using template recursion, but
all_of_t/any_of_t can also make use of it.

Type alias for alias classes with members didn't work properly: space
was only allocated for sizeof(type), but if we want to be able to put a
type_alias instance there, we need sizeof(type_alias), but
sizeof(type_alias) > sizeof(type) whenever type_alias has members.