[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_class.cpp

@@ -25,8 +25,6 @@
 PYBIND11_WARNING_DISABLE_MSVC(4324)
 //     warning C4324: structure was padded due to alignment specifier
 
-namespace {
-
 // test_brace_initialization
 struct NoBraceInitialization {
     explicit NoBraceInitialization(std::vector<int> v) : vec{std::move(v)} {}
@@ -36,17 +34,6 @@ struct NoBraceInitialization {
     std::vector<int> vec;
 };
 
-// test_mismatched_holder
-struct MismatchBase1 {};
-struct MismatchDerived1 : MismatchBase1 {};
-struct MismatchBase2 {};
-struct MismatchDerived2 : MismatchBase2 {};
-
-// test_multiple_instances_with_same_pointer
-struct SamePointer {};
-
-} // namespace
-
 namespace test_class {
 namespace pr4220_tripped_over_this { // PR #4227
 
@@ -67,12 +54,6 @@ void bind_empty0(py::module_ &m) {
 } // namespace pr4220_tripped_over_this
 } // namespace test_class
 
-PYBIND11_TYPE_CASTER_BASE_HOLDER(MismatchBase1, std::shared_ptr<MismatchBase1>)
-PYBIND11_TYPE_CASTER_BASE_HOLDER(MismatchDerived1, std::unique_ptr<MismatchDerived1>)
-PYBIND11_TYPE_CASTER_BASE_HOLDER(MismatchBase2, std::unique_ptr<MismatchBase2>)
-PYBIND11_TYPE_CASTER_BASE_HOLDER(MismatchDerived2, std::shared_ptr<MismatchDerived2>)
-PYBIND11_TYPE_CASTER_BASE_HOLDER(SamePointer, std::unique_ptr<SamePointer>)
-
 TEST_SUBMODULE(class_, m) {
     m.def("obj_class_name", [](py::handle obj) { return py::detail::obj_class_name(obj.ptr()); });
 
@@ -108,6 +89,16 @@ TEST_SUBMODULE(class_, m) {
         .def_static("__new__",
                     [](const py::object &) { return NoConstructorNew::new_instance(); });
 
+    // test_pass_unique_ptr
+    struct ToBeHeldByUniquePtr {};
+    py::class_<ToBeHeldByUniquePtr, std::unique_ptr<ToBeHeldByUniquePtr>>(m, "ToBeHeldByUniquePtr")
+        .def(py::init<>());
+#ifdef PYBIND11_HAVE_INTERNALS_WITH_SMART_HOLDER_SUPPORT
+    m.def("pass_unique_ptr", [](std::unique_ptr<ToBeHeldByUniquePtr> &&) {});
+#else
+    m.attr("pass_unique_ptr") = py::none();
+#endif
+
     // test_inheritance
     class Pet {
     public:
@@ -221,6 +212,12 @@ TEST_SUBMODULE(class_, m) {
     m.def("as_type", [](const py::object &ob) { return py::type(ob); });
 
     // test_mismatched_holder
+    struct MismatchBase1 {};
+    struct MismatchDerived1 : MismatchBase1 {};
+
+    struct MismatchBase2 {};
+    struct MismatchDerived2 : MismatchBase2 {};
+
     m.def("mismatched_holder_1", []() {
         auto mod = py::module_::import("__main__");
         py::class_<MismatchBase1, std::shared_ptr<MismatchBase1>>(mod, "MismatchBase1");
@@ -520,6 +517,7 @@ TEST_SUBMODULE(class_, m) {
         .def("throw_something", &PyPrintDestructor::throw_something);
 
     // test_multiple_instances_with_same_pointer
+    struct SamePointer {};
     static SamePointer samePointer;
     py::class_<SamePointer, std::unique_ptr<SamePointer, py::nodelete>>(m, "SamePointer")
         .def(py::init([]() { return &samePointer; }));
@@ -619,18 +617,12 @@ CHECK_NOALIAS(8);
     static_assert(std::is_same<typename DoesntBreak##N::holder_type,                              \
                                std::TYPE##_ptr<BreaksBase<(N)>>>::value,                          \
                   "DoesntBreak" #N " has wrong holder_type!")
-#define CHECK_SMART_HOLDER(N)                                                                     \
-    static_assert(std::is_same<typename DoesntBreak##N::holder_type, py::smart_holder>::value,    \
-                  "DoesntBreak" #N " has wrong holder_type!")
 CHECK_HOLDER(1, unique);
 CHECK_HOLDER(2, unique);
 CHECK_HOLDER(3, unique);
-#ifndef PYBIND11_USE_SMART_HOLDER_AS_DEFAULT
+#ifndef PYBIND11_ACTUALLY_USING_SMART_HOLDER_AS_DEFAULT
 CHECK_HOLDER(4, unique);
 CHECK_HOLDER(5, unique);
-#else
-CHECK_SMART_HOLDER(4);
-CHECK_SMART_HOLDER(5);
 #endif
 CHECK_HOLDER(6, shared);
 CHECK_HOLDER(7, shared);