Towards #5479

Use __is_trivially_equality_comparable starting in Clang 19, with escape hatch.
Distinguish memcmp and search-like vector algorithms, as the latter need 1, 2, 4, or 8 element size.

If this takes on, may improve mismatch to vectorize for odd element size too.
It takes division, but it can be done in the header, where it will be optimized to big multiplication.

Also can look into _Vector_alg_in_find_is_safe.

I'm not sure though:

• We can't have enums with this actually https://godbolt.org/z/33bsEvq1b
• For structs need defaulted operator==
  • Which means pair<int, int> or chrono::duration<uint64_t, pico> will not be automatically engaged
• On the other hand, here came member object pointer and member function pointer, which weren't invited actually
• This all works for Clang only, making Clang behavior w.r.t. vectorization very different