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This PR brings the std::array<> caster in sync with the other STL type
casters: to accept an arbitrary sequence as input (rather than a list,
which is too restrictive).

* Fix for Issue #1258

list_caster::load method will now check for a Python string and prevent its automatic conversion to a list.
This should fix the issue "pybind11/stl.h converts string to vector<string> #1258" (https://github.com/pybind/pybind11/issues/1258)

* Added tests for fix of issue #1258

* Changelog: stl string auto-conversion

* stl.h: propagate return value policies to type-specific casters

Return value policies for containers like those handled in in 'stl.h'
are currently broken.

The problem is that detail::return_value_policy_override<C>::policy()
always returns 'move' when given a non-pointer/reference type, e.g.
'std::vector<...>'.

This is sensible behavior for custom types that are exposed via
'py::class_<>', but it does not make sense for types that are handled by
other type casters (STL containers, Eigen matrices, etc.).

This commit changes the behavior so that
detail::return_value_policy_override only becomes active when the type
caster derives from type_caster_generic.

Furthermore, the override logic is called recursively in STL type
casters to enable key/value-specific behavior.

To avoid an ODR violation in the test suite while testing
both `stl.h` and `std_bind.h` with `std::vector<bool>`,
the `py::bind_vector<std::vector<bool>>` test is moved to
the secondary module (which does not include `stl.h`).

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.

This udpates all the remaining tests to the new test suite code and
comment styles started in #898.  For the most part, the test coverage
here is unchanged, with a few minor exceptions as noted below.

- test_constants_and_functions: this adds more overload tests with
  overloads with different number of arguments for more comprehensive
  overload_cast testing.  The test style conversion broke the overload
  tests under MSVC 2015, prompting the additional tests while looking
  for a workaround.

- test_eigen: this dropped the unused functions `get_cm_corners` and
  `get_cm_corners_const`--these same tests were duplicates of the same
  things provided (and used) via ReturnTester methods.

- test_opaque_types: this test had a hidden dependence on ExampleMandA
  which is now fixed by using the global UserType which suffices for the
  relevant test.

- test_methods_and_attributes: this required some additions to UserType
  to make it usable as a replacement for the test's previous SimpleType:
  UserType gained a value mutator, and the `value` property is not
  mutable (it was previously readonly).  Some overload tests were also
  added to better test overload_cast (as described above).

- test_numpy_array: removed the untemplated mutate_data/mutate_data_t:
  the templated versions with an empty parameter pack expand to the same
  thing.

- test_stl: this was already mostly in the new style; this just tweaks
  things a bit, localizing a class, and adding some missing
  `// test_whatever` comments.

- test_virtual_functions: like `test_stl`, this was mostly in the new
  test style already, but needed some `// test_whatever` comments.
  This commit also moves the inherited virtual example code to the end
  of the file, after the main set of tests (since it is less important
  than the other tests, and rather length); it also got renamed to
  `test_inherited_virtuals` (from `test_inheriting_repeat`) because it
  tests both inherited virtual approaches, not just the repeat approach.

PR #936 broke the ability to return a pointer to a stl container (and,
likewise, to a tuple) because the added deduced type matched a
non-const pointer argument: the pointer-accepting `cast` in
PYBIND11_TYPE_CASTER had a `const type *`, which is a worse match for a
non-const pointer than the universal reference template #936 added.

This changes the provided TYPE_CASTER cast(ptr) to take the pointer by
template arg (so that it will accept either const or non-const pointer).
It has two other effects: it slightly reduces .so size (because many
type casters never actually need the pointer cast at all), and it allows
type casters to provide their untemplated pointer `cast()` that will
take precedence over the templated version provided in the macro.

This updates the std::tuple, std::pair and `stl.h` type casters to
forward their contained value according to whether the container being
cast is an lvalue or rvalue reference.  This fixes an issue where
subcaster casts were always called with a const lvalue which meant
nested type casters didn't have the desired `cast()` overload invoked.
For example, this caused Eigen values in a tuple to end up with a
readonly flag (issue #935) and made it impossible to return a container
of move-only types (issue #853).

This fixes both issues by adding templated universal reference `cast()`
methods to the various container types that forward container elements
according to the container reference type.

`nullptr` is not expected to work in this case.