Convert interval sets inside TypeSet/ValueTypeSet in general sets (#102)

* 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

lib/cretonne/src/lib.rs

@@ -16,6 +16,7 @@ pub mod dbg;
 pub mod entity_ref;
 
 pub mod binemit;
+pub mod bitset;
 pub mod dominator_tree;
 pub mod entity_list;
 pub mod entity_map;