Add function for which characters an FSM's input must contain to match

[silentbicycle]

It would be useful to have a function that populates a u64bitset with which characters must appear in the input to possibly match an FSM. This wouldn't track the relative order of those characters occurring, just that if bitset[N] is set and character/octet N does not appear in the input at all, it can never match.

This could be based on either the regex AST or walking the FSM. With the FSM it would benefit from fsm_trims's shortest_end_distance analysis -- I think the intersection of the bitsets from all single-label edges on the shortest reverse path(s) from every endstate to the start would be one way to do it (for paths that cannot be skipped via a longer route), though not necessarily the simplest/cheapest.