Use hashset data structure in bf_set

[pzmarzly]

Currently bf_set uses linked lists under the hood, and the operations are much slower than they could be.

Let's write new bf_set implementation, that will use simple hashsets. API can stay the same. Let's try avoiding pulling in any dependencies, hsearch from glibc should be enough.

Initially let's have it live alongside bf_set, maybe call it bf_hashset. Once we have it tested, we can try swapping the old implementation out.

[pzmarzly]

Maybe bf_fnv1a from bf22d30 would be better than hsearch.