Using type_caster_base<mpty>::src_and_type directly, removing copy. Also renaming one cast to cast_const_raw_ptr, for clarity.

Open question, with respect to the original code:
https://github.com/pybind/pybind11/blob/76a160070b369f8d82b945c97924227e8b835c94/include/pybind11/pybind11.h#L1510
To me it looks like the exact situation marked as `std::shared_ptr<Good> gp1 = not_so_good.getptr();` here: https://en.cppreference.com/w/cpp/memory/enable_shared_from_this
The comment there is: `// undefined behavior (until C++17) and std::bad_weak_ptr thrown (since C++17)`
Does the existing code have UB pre C++17?

I'll leave handling of enable_shared_from_this for later, as the need arises.

See new bullet point in comment section near the top.

The variable was also renamed to reflect its function more accurately.

Copying type_caster_generic::cast into type_caster<mpty> as-is (preparation for handling smart pointers).

static cast handle for rtrn_valu works by simply dropping in code from type_caster_base (marked with comments). rtrn_rref raises a RuntimeError, to be investigated.

static cast handle for rtrn_cref works by simply dropping in code from type_caster_base (marked with comments). rtrn_mref and rtrn_mptr work via const_cast (to add const).

static cast handle for rtrn_cptr works by simply dropping in code from type_caster_base (marked with comments).

Systematically setting mtxt for all rtrn_mpty_* functions (preparation, the values are not actually used yet).

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.

Pure copy of `class class_` implementation in pybind11.h (master commit 98f1bbb8).