- String hashing is not inlinable, so it can use _Hasher._core operations directly. Remove custom buffering.
- Speed up ASCII hashing by as much as 5.5x by feeding the storage buffer directly into hasher in a single go.
- For other strings, just feed the UTF-8 encoding of the normalized string to the hasher; don't switch to UTF-16 at the first non-ASCII scalar. (Doing that would make the hash encoding of some string sequences ambiguous, leading to artificial collisions.)
- Add a single unconditional terminator byte, 0xFF. It's not a valid UTF-8 code unit, so it won't ever occur within a normalized string encoding.
