[smart_holder] Bake smart_holder functionality into class_ and type_caster_base (#5257)

* Put bakein branch @ 18b72c0 on top of smart_holder branch:

Commands used:

```
git checkout bakein
git diff smart_holder > ~/zd
git checkout smart_holder
git checkout -b bakein_sh
patch -p 1 < ~/zd
git checkout smart_holder \
MANIFEST.in \
README.rst \
README_smart_holder.rst \
docs/advanced/smart_ptrs.rst \
ubench/holder_comparison.cpp \
ubench/holder_comparison.py \
ubench/holder_comparison_extract_sheet_data.py \
ubench/number_bucket.h \
ubench/python/number_bucket.clif
git add -A
```

* Add back README_smart_holder.rst in tests/extra_python_package/test_files.py

* Restore smart_holder_poc.h as-is on smart_holder branch (i.e. undo `PYBIND11_SMART_HOLDER_PADDING`, which was meant for stress-testing only).

* Insert `std::move()` as suggested by @laramiel

* `property_cpp_function_sh_*` named specializations, as suggested by @laramiel (#5257 (comment))

* Call `property_cpp_function_classic` member functions, rather than inlining the implementations.

* Use `PYBIND11_HAVE_INTERNALS_WITH_SMART_HOLDER_SUPPORT` in holder_comparison.cpp (holder_comparison.py is NOT changed accordingly in this commit, i.e. can still only be run if the smart_holder functionality is available).

* Systematically rename `loaded_as` to `load_as` (`shared_ptr`, `unique_ptr`) as suggested by @laramiel

* Make change as suggested by @laramiel. This makes it much more obvious that the latest implementation of `smart_holder_from_unique_ptr()` accepts all existing `return_value_policy` enum values except `copy`.

* Resolve `BAKEIN_WIP: Rewrite comment.` for `property_cpp_function_*` specializations.

* Resolve `BAKEIN_WIP: Add comment to explain: This is meant for stress-testing only.`

* Resolve all remaining BAKEIN_WIP (in pybind11/cast.h).

Leave only two pairs of SMART_HOLDER_BAKEIN_FOLLOW_ON comments: refactoring of copyable_holder_caster, move_only_holder_caster. This is best left until after the smart_holder branch is merged into the master branch.

* Remove obsolete `using holder_type = smart_holder;` in `load_helper`

* Add SMART_HOLDER_BAKEIN_FOLLOW_ON comment for `internals::default_holder`

* README_smart_holder.rst update (line count reduced from 356 to 123).

Author: rwgk

CMakeLists.txt

@@ -154,10 +154,9 @@ set(PYBIND11_HEADERS
     include/pybind11/detail/init.h
     include/pybind11/detail/internals.h
     include/pybind11/detail/smart_holder_poc.h
-    include/pybind11/detail/smart_holder_sfinae_hooks_only.h
-    include/pybind11/detail/smart_holder_type_casters.h
     include/pybind11/detail/type_caster_base.h
     include/pybind11/detail/typeid.h
+    include/pybind11/detail/using_smart_holder.h
     include/pybind11/detail/value_and_holder.h
     include/pybind11/attr.h
     include/pybind11/buffer_info.h