Renaming type_caster_type_is_smart_holder_type_caster -> wrapped_type_uses_smart_holder_type_caster for clarity.

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Tested using github.com/rwgk/pybind11_scons and Google-internal build system.
Sorry, no cmake support at the moment.

First results: https://docs.google.com/spreadsheets/d/1InapCYws2Gt-stmFf_Bwl33eOMo3aLE_gc9adveY7RU/edit#gid=0

Splitting out smart_holder_type_casters again, into new detail/smart_holder_type_casters_inline_include.h.

Systematically adding SMART_HOLDER_WIP tag. Removing minor UNTESTED tags (only the throw are not actually exercised, investing time there has a high cost but very little benefit).

Adding test_class_sh_factory_constructors.cpp to tests/CMakeLists.txt (fixes oversight, this should have been added long before).

Adding construct() overloads for constructing smart_holder from alias unique_ptr, shared_ptr returns.

Reverting commit 63495313 by un-commenting `const` in `def_buffer(...)`. To make this possible, `operator T const&` and `operator T const*` in `smart_holder_type_caster` need to be marked as `const` member functions.

Removing unused smart_holder_type_caster_load::loaded_as_rvalue_ref (it was an oversight that it was not removed with commit 23036a45).

Systematically changing std::enable_if back to detail::enable_if_t, effectively reverting commit 5d4b6890.

Systematically replacing `detail::enable_if_t<...smart_holder...>` with `typename std::enable_if<...smart_holder...>::type`. Attempt to work around MSVC 2015 issues, to be tested via GitHub CI. The idea for this change originates from this comment: https://github.com/pybind/pybind11/issues/1616#issuecomment-444536813

Also removing `operator T()` from smart_holder_type_caster, to fix gcc compilation errors. The only loss is pass_rref in test_class_sh_basic.

Using @pytest.mark.parametrize to run each assert separately (to see all errors, not just the first).

Fixing unfortunate editing mishap. This reverts the last remaining test manipulation in commit 249df7cb and makes all existing unit tests pass with smart_holder as default holder.

Removing `operator T&&() &&` from smart_holder_type_caster, for compatibility with the behavior of type_caster_base. Enables reverting 2 of 3 test manipulations applied under commit 249df7cb. The manipulation in test_factory_constructors.py is NOT reverted in this commit.

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Adding copy constructor and move constructor tracking to atyp. Preparation for a follow-up change in smart_holder_type_caster, to make this test sensitive to the changing behavior.

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