[libc++][NFC] Reorganize hash.h

- Add missing _LIBCPP_HIDE_FROM_ABI
- Implement inline functions in the class to simplify the code
- Add missing `const` to `operator()`
- Move _LIBCPP_DISABLE_UBSAN_UNSIGNED_INTEGER_CHECK to the usual location for function attributes

Differential Revision: https://reviews.llvm.org/D143668