WG21-P2652R2 Disallowing User Specialization Of allocator_traits

No feature-test macro is mentioned.

In addition to this paper's observable changes, there's the question that @frederick-vs-ja raised on Discord - what to do about:

If a program declares an explicit or partial specialization of allocator_traits, the program is ill-formed, no diagnostic required.

We could do nothing (allowing squirrelly programs to be bad kitties 🐿️ 😼), or we could implement enforcement (with possible throughput impact and extensive codebase edits).