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Retrieving smart_holder pointer in type_caster<std::unique_ptr<mpty[ const]>>::load, and using it cast_op operators.

Retrieving smart_holder pointer in type_caster<std::shared_ptr<mpty[ const]>>::load, and using it cast_op operators.

classh.h: renaming of class_ to classh + namespace; forking test_classh_wip from test_type_caster_bare_interface_demo.

minor test_private_first_base.cpp simplification (after discovering that this can be wrapped with Boost.Python, using boost::noncopyable)

(This demo does NOT involve smart pointers at all, unlike the otherwise similar test_smart_ptr_private_first_base.)

Based on https://godbolt.org/z/4fdjaW by jorgbrown@ (thanks Jorg!).

https://github.com/rwgk/pybind11/tree/xxx_value_ptr_xxx_holder

Systematically exercising casting between shared_ptr<base>, shared_ptr<derived>.

https://github.com/rwgk/pybind11/tree/xxx_value_ptr_xxx_holder

Systematically exercising returning and passing unique_ptr<T>, shared_ptr<T>
with unique_ptr, shared_ptr holder.

Observations:

test_holder_unique_ptr:
  make_unique_pointee  OK
  pass_unique_pointee  BUILD_FAIL (as documented)
  make_shared_pointee  Abort free(): double free detected
  pass_shared_pointee  RuntimeError: Unable to load a custom holder type from a default-holder instance

test_holder_shared_ptr:
  make_unique_pointee  Segmentation fault (#1138)
  pass_unique_pointee  BUILD_FAIL (as documented)
  make_shared_pointee  OK
  pass_shared_pointee  OK

moving prototype code to pybind11/vptr_holder.h, adding type_caster specialization to make the bindings involving unique_ptr passing compile, but load and cast implementations are missing

See also: https://github.com/pybind/pybind11/issues/2583

Does not build with upstream master or
https://github.com/pybind/pybind11/pull/2047, but builds with
https://github.com/RobotLocomotion/pybind11 and almost runs:

```
Running tests in directory "/usr/local/google/home/rwgk/forked/EricCousineau-TRI/pybind11/tests":
================================================================================= test session starts =================================================================================
platform linux -- Python 3.8.5, pytest-5.4.3, py-1.9.0, pluggy-0.13.1
rootdir: /usr/local/google/home/rwgk/forked/EricCousineau-TRI/pybind11/tests, inifile: pytest.ini
collected 2 items

test_unique_ptr_member.py .F                                                                                                                                                    [100%]

====================================================================================== FAILURES =======================================================================================
_____________________________________________________________________________ test_pointee_and_ptr_owner ______________________________________________________________________________

    def test_pointee_and_ptr_owner():
        obj = m.pointee()
        assert obj.get_int() == 213
        m.ptr_owner(obj)
        with pytest.raises(ValueError) as exc_info:
>           obj.get_int()
E           Failed: DID NOT RAISE <class 'ValueError'>

test_unique_ptr_member.py:17: Failed
============================================================================= 1 failed, 1 passed in 0.06s =============================================================================
```

* Changing pybind11::str to exclusively hold PyUnicodeObject