P3471R4 Standard Library Hardening

[StephanTLavavej]

WG21-P3471R4 Standard Library Hardening

Feature-test macros (expected):

#define __cpp_lib_hardened_array 202502L
#define __cpp_lib_hardened_basic_string 202502L
#define __cpp_lib_hardened_basic_string_view 202502L
#define __cpp_lib_hardened_bitset 202502L
#define __cpp_lib_hardened_deque 202502L
#define __cpp_lib_hardened_expected 202502L
#define __cpp_lib_hardened_forward_list 202502L
#define __cpp_lib_hardened_inplace_vector 202502L
#define __cpp_lib_hardened_list 202502L
#define __cpp_lib_hardened_mdspan 202502L
#define __cpp_lib_hardened_optional 202502L
#define __cpp_lib_hardened_span 202502L
#define __cpp_lib_hardened_valarray 202502L
#define __cpp_lib_hardened_vector 202502L

Note: We're focused on implementing the remaining library-only features in C++23. Until that's done, we will NOT be accepting PRs for C++26 features.

I am substantially implementing this with #5274, but R4 was specified to use contracts, so this feature is blocked by #5286.