hashtable: add generic hashtable

SDL_HashTable uses callbacks to calculate hash, compare keys and destroy items. Compiler doesn't know anything about functions that hashtable is going to call, so some optimizations can't be applied. But in almost all cases we know the functions to be called at compile time.

Generic hashtable is a reusable header that generates hashtable implementation for concrete hash, compare and destroy functions. It allows them to be inlined by the compiler, resulting in 1.5-4x performance boost compared to SDL_HashTable. The implementation is taken from SDL_HashTable, with a few improvements:

- Lowered max load factor to 128 (50%). Seems to be a good default value, as find is unlikely to load more than one cache line. The value also can be overriden by setting SDL_HASHTABLE_MAX_LOAD_FACTOR.

- Removed live bit field, using probe_len = 0 to indicate whether the item is live instead. Seems to be a bit faster, as CPU doesn't need to reset live bit each time probe_len is accessed.

- Removed max_probe_len. Unneeded, as we compare to probe_len of each item we visit, which is a stronger break condition.

Author: mechakotik