Refactor Compact.c:
Refactor Compact.c:

- Remove forward declarations
- Introduce UNTAG_PTR and GET_PTR_TAG for dealing with pointer tags
  without having to cast arguments to StgClosure*
- Remove dead code
- Use W_ instead of StgWord
- Use P_ instead of StgPtr

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc