Clean ups:

   - reduce the namespace pollution of StgTypes.h, it doesn't define
     the shorthand versions any more (W_, I_ etc.).  These are moved into
     Stg.h.  StgTypes.h also defines StgClosure as an "opaque" struct.

   - RtsAPI.h is now standalone, and includes HsFFI.h and thereby
     config.h & StgTypes.h.  Now we don't need to #include "Stg.h" in
     *_stub.c.

   - all the rts_mkXXXX and rts_getXXXX functions are defined in terms
     of the HsXXXX types rather than random C types (this fixes some
     potential bugs in our foreign export support).

   - added HsWord type, to match StgWord.  The Haskell version of this
     type isn't "documented", but perhaps it should be.