[libc++][ranges]Refactor copy{,_backward} and move{,_backward}

Instead of using `reverse_iterator`, share the optimization between the 4 algorithms. The key observation here that `memmove` applies to both `copy` and `move` identically, and to their `_backward` versions very similarly. All algorithms now follow the same pattern along the lines of:
```
if constexpr (can_memmove<InIter, OutIter>) {
  memmove(first, last, out);
} else {
  naive_implementation(first, last, out);
}
```
A follow-up will delete `unconstrained_reverse_iterator`.

This patch removes duplication and divergence between `std::copy`, `std::move` and `std::move_backward`. It also improves testing:
- the test for whether the optimization is used only applied to `std::copy` and, more importantly, was essentially a no-op because it would still pass if the optimization was not used;
- there were no tests to make sure the optimization is not used when the effect would be visible.

Differential Revision: https://reviews.llvm.org/D130695

libcxx/include/algorithm

@@ -1708,7 +1708,6 @@ template <class BidirectionalIterator, class Compare>
 #include <__config>
 #include <__debug>
 #include <cstddef>
-#include <cstring>
 #include <type_traits>
 #include <version>
 
@@ -1917,6 +1916,7 @@ template <class BidirectionalIterator, class Compare>
 
 #if !defined(_LIBCPP_REMOVE_TRANSITIVE_INCLUDES) && _LIBCPP_STD_VER <= 20
 #  include <atomic>
+#  include <cstring>
 #  include <iterator>
 #  include <memory>
 #  include <stdexcept>