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Closes #1048, closes #1052.

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PR #880 changed the implementation of keep_alive to avoid weak
references when the nurse is pybind11-registered, but the documentation
didn't get updated to match.

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There are two separate additions:

1. `py::hash(obj)` is equivalent to the Python `hash(obj)`.
2. `.def(hash(py::self))` registers the hash function defined by
   `std::hash<T>` as the Python hash function.

The warning is shown at module initialization time (on import, not
when the functions are called). It's only visible when compiled in
debug mode.

This is analogous to `py::init()` vs `__init__` + placement-new.
`py::pickle()` reuses most of the implementation details of `py::init()`.

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Fixes one small variable name typo, and two instances where `py::arg().nocopy()` is used, where I think it should be `py::arg().noconvert()` instead. Probably `nocopy()` was the old/original name for it and then it was changed.

The main point of `py::module_local` is to make the C++ -> Python cast
unique so that returning/casting a C++ instance is well-defined.
Unfortunately it also makes loading unique, but this isn't particularly
desirable: when an instance contains `Type` instance there's no reason
it shouldn't be possible to pass that instance to a bound function
taking a `Type` parameter, even if that function is in another module.

This commit solves the issue by allowing foreign module (and global)
type loaders have a chance to load the value if the local module loader
fails.  The implementation here does this by storing a module-local
loading function in a capsule in the python type, which we can then call
if the local (and possibly global, if the local type is masking a global
type) version doesn't work.

This allows you to use:

    cls.def(py::init(&factory_function));

where `factory_function` returns a pointer, holder, or value of the
class type (or a derived type).  Various compile-time checks
(static_asserts) are performed to ensure the function is valid, and
various run-time type checks where necessary.

Some other details of this feature:
- The `py::init` name doesn't conflict with the templated no-argument
  `py::init<...>()`, but keeps the naming consistent: the existing
  templated, no-argument one wraps constructors, the no-template,
  function-argument one wraps factory functions.
- If returning a CppClass (whether by value or pointer) when an CppAlias
  is required (i.e. python-side inheritance and a declared alias), a
  dynamic_cast to the alias is attempted (for the pointer version); if
  it fails, or if returned by value, an Alias(Class &&) constructor
  is invoked.  If this constructor doesn't exist, a runtime error occurs.
- for holder returns when an alias is required, we try a dynamic_cast of
  the wrapped pointer to the alias to see if it is already an alias
  instance; if it isn't, we raise an error.
- `py::init(class_factory, alias_factory)` is also available that takes
  two factories: the first is called when an alias is not needed, the
  second when it is.
- Reimplement factory instance clearing.  The previous implementation
  failed under python-side multiple inheritance: *each* inherited
  type's factory init would clear the instance instead of only setting
  its own type value.  The new implementation here clears just the
  relevant value pointer.
- dealloc is updated to explicitly set the leftover value pointer to
  nullptr and the `holder_constructed` flag to false so that it can be
  used to clear preallocated value without needing to rebuild the
  instance internals data.
- Added various tests to test out new allocation/deallocation code.
- With preallocation now done lazily, init factory holders can
  completely avoid the extra overhead of needing an extra
  allocation/deallocation.
- Updated documentation to make factory constructors the default
  advanced constructor style.
- If an `__init__` is called a second time, we have two choices: we can
  throw away the first instance, replacing it with the second; or we can
  ignore the second call.  The latter is slightly easier, so do that.

* Doxygen needs `RECURSIVE = YES` in order to parse the `detail` subdir.

* The `-W` warnings-as-errors option for sphinx doesn't work with the
  makefile build. Switched to calling sphinx directly.

* Fix "citation [cppimport] is not referenced" warning.

This updates the compilation to always apply hidden visibility to
resolve the issues with default visibility causing problems under debug
compilations.  Moreover using the cmake property makes it easier for a
caller to override if absolutely needed for some reason.

For `pybind11_add_module` we use cmake to set the property; for the
targets, we append to compilation option to non-MSVC compilers.

In C++11 mode, `boost::apply_visitor` requires an explicit `result_type`.
This also adds optional tests for `boost::variant` in C++11/14, if boost
is available. In C++17 mode, `std::variant` is tested instead.

* Ensure :ref: for virtual_and_inheritance is parsed.

* Add quick blurb about __init__ with inherited types.

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boost::apply_visitor accepts its arguments by non-const lvalue
reference, which fails to bind to an rvalue reference.  Change the
example to remove the argument forwarding.

Attempting to mix py::module_local and non-module_local classes results
in some unexpected/undesirable behaviour:

- if a class is registered non-local by some other module, a later
  attempt to register it locally fails.  It doesn't need to: it is
  perfectly acceptable for the local registration to simply override
  the external global registration.
- going the other way (i.e. module `A` registers a type `T` locally,
  then `B` registers the same type `T` globally) causes a more serious
  issue: `A.T`'s constructors no longer work because the `self` argument
  gets converted to a `B.T`, which then fails to resolve.

Changing the cast precedence to prefer local over global fixes this and
makes it work more consistently, regardless of module load order.

This commit adds a `py::module_local` attribute that lets you confine a
registered type to the module (more technically, the shared object) in
which it is defined, by registering it with:

    py::class_<C>(m, "C", py::module_local())

This will allow the same C++ class `C` to be registered in different
modules with independent sets of class definitions.  On the Python side,
two such types will be completely distinct; on the C++ side, the C++
type resolves to a different Python type in each module.

This applies `py::module_local` automatically to `stl_bind.h` bindings
when the container value type looks like something global: i.e. when it
is a converting type (for example, when binding a `std::vector<int>`),
or when it is a registered type itself bound with `py::module_local`.
This should help resolve potential future conflicts (e.g. if two
completely unrelated modules both try to bind a `std::vector<int>`.
Users can override the automatic selection by adding a
`py::module_local()` or `py::module_local(false)`.

Note that this does mildly break backwards compatibility: bound stl
containers of basic types like `std::vector<int>` cannot be bound in one
module and returned in a different module.  (This can be re-enabled with
`py::module_local(false)` as described above, but with the potential for
eventual load conflicts).

Resolves #645.

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