* Convert TypeSet fields to sets; Add BitSet<T> type to rust; Encode ValueTypeSets using BitSet; (still need mypy cleanup)
* nits
* cleanup nits
* forgot mypy type annotations
* rustfmt fixes
* Round 1 comments: filer b2, b4; doc comments in python; move bitset in its own toplevel module; Use Into<u32>
* fixes
* Revert comment to appease rustfmt