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.

DEBUGGING_MSVC_2015: simplification (from compiler perspective) of struct is_smart_holder_type_caster implementation.

Uncommenting `#define PYBIND11_USE_SMART_HOLDER_AS_DEFAULT` to run GitHub CI with smart_holder as default. NOT meant for merging.

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.

[skip ci]

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.

[skip ci]

This version of the code is ASAN clean with unique_ptr or smart_holder as the default.

This change needs to be reverted after adopting the existing move-only-if-refcount-is-1
logic used by type_caster_base.

Adding return_value_policy::move to permissible policies for unique_ptr returns. New stats for all tests combined: 3 failed, 467 passed.

Changing std::shared_ptr pointer/reference to const pointer/reference. New stats for all tests combined: 4 failed, 466 passed.