Library constexpr-ification (C++26)

Makes stable_sort, stable_partition and inplace_merge constexpr.

Adds std::is_within_lifetime to check which union member is active during constant evaluation.

Makes std::atomic and atomic_ref constexpr.

Makes uninitialized_default_construct constexpr.

Wording for constexpr specialized memory algorithms (uninitialized_copy etc.).

Makes the remaining containers and adaptors (map, set, deque, list, flat_*, unordered_*…) constexpr.

Makes the standard exception types (std::exception, logic_error…) constexpr.

Makes std::shared_ptr, weak_ptr and related functions constexpr.

Extends std::is_within_lifetime to more cases.

Allows std::uncaught_exceptions() and current_exception() in constant evaluation in a conservative way.

Adds pointer-tagging utilities usable in constant evaluation.