- Add SmallFrozenDictionary/Set which don't do any hashing and simply iterate through arrays,
testing each element

- Add ClosedRangeInt32FrozenDictionary/Set which are used when the keys are in a
closed integer range.

- Add SparseRangeInt32FrozenSet which uses a bit vector for storage.

FrozenDictionary/Set each have some constants defined to drive the heuristics that
decide when to use the small and sparse collection types. I did some preliminary tuning
of these, but this could use a detailed benchmark to pick the right values. I'll leave
the fine-tuning to a separate PR.