[libc++] Optimize char_traits a bit (#72799)

This implements two kinds of optimizations. Specifically
- `char_traits<char8_t>` uses `char` code paths; these are heavily
  optimized and the operations are equivalent
- `char16_t` and `char32_t` `find` uses `std::find` to forward to
  `wmemchr` if they have the same size