VectorSet cleanup

VectorSet is the only concrete implementation of the abstract base class Set in the libadt folder. This is a utility used mainly by compiler code, although there are some uses in GC code.

- Extravagant use of operator overloading (<<= adds items to the set, >>= removes them)
- Plenty of unused methods
- Since VectorSet is the only implementation in the code base, Set is unnecessary
- Various method names in conflict with HotSpot standard

Cleaning up and removing dead code gets rid of around ~700 lines of code.