1. use `unsigned int` to store the codepoints, this avoids waste on
common case where `long` is 64bits. and POSIX guarantees `int` to be
at least 32bits so there's no risk of truncation.
2. since switching to `unsigned int` cuts down the memory requirement by
half, double the cache size from 64 to 128.
3. instead of a linear search, use a simple hash-table for O(1) lookups.
ref.
https://git.suckless.org/dmenu/commit/7ab0cb5ef0e19352fc5d64ae0d57a5cf4540acbf.html