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

* Put bakein branch @ 18b72c0ffa 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 (https://github.com/pybind/pybind11/pull/5257#discussion_r1688346807)

* 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).

tests/test_classh_mock.cpp

@@ -1,18 +1,20 @@
 #include "pybind11_tests.h"
 
-// The main purpose of this test is to ensure the suggested BOILERPLATE code block below is
-// correct.
+// The main purpose of this test was to ensure that the suggested
+// BOILERPLATE code block (NOW DEPRECATED!) block below is correct.
 
 // Copy this block of code into your project.
 // Replace FOOEXT with the name of your project.
-// BOILERPLATE BEGIN
+// BOILERPLATE BEGIN DEPRECATED DEPRECATED DEPRECATED DEPRECATED DEPRECATED
 #ifdef FOOEXT_USING_PYBIND11_SMART_HOLDER
 #    include <pybind11/smart_holder.h>
 #else
 #    include <pybind11/pybind11.h>
 PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE)
+#    ifndef PYBIND11_HAVE_INTERNALS_WITH_SMART_HOLDER_SUPPORT
 template <typename type_, typename... options>
 using classh = class_<type_, options...>;
+#    endif
 PYBIND11_NAMESPACE_END(PYBIND11_NAMESPACE)
 #    ifndef PYBIND11_SH_AVL
 #        define PYBIND11_SH_AVL(...) std::shared_ptr<__VA_ARGS__> // "Smart_Holder if AVaiLable"
@@ -27,7 +29,7 @@ PYBIND11_NAMESPACE_END(PYBIND11_NAMESPACE)
 #        define PYBIND11_TYPE_CASTER_BASE_HOLDER(...)
 #    endif
 #endif
-// BOILERPLATE END
+// BOILERPLATE END DEPRECATED DEPRECATED DEPRECATED DEPRECATED DEPRECATED
 
 namespace {
 struct FooUc {};
@@ -37,8 +39,8 @@ struct FooSc {};
 struct FooSp {};
 } // namespace
 
-PYBIND11_SMART_HOLDER_TYPE_CASTERS(FooUp)
-PYBIND11_SMART_HOLDER_TYPE_CASTERS(FooSp)
+PYBIND11_SMART_HOLDER_TYPE_CASTERS(FooUp) // DEPRECATED
+PYBIND11_SMART_HOLDER_TYPE_CASTERS(FooSp) // DEPRECATED
 
 PYBIND11_TYPE_CASTER_BASE_HOLDER(FooSa, std::shared_ptr<FooSa>)