PrimRep Cleanup

   - Remove all PrimReps which were just different flavours of
     PtrRep.  Now, everything which is a pointer to a closure of
     some kind is always a PtrRep.

   - Three of the deleted PrimReps, namely ArrayRep, ByteArrayRep,
     and ForeignObj rep, had a subtle reason for their existence:
     the abstract C pretty-printer(!) used them to decide whether
     to apply a shim to an outgoing C-call argument: a ByteArrayRep
     argument would be adjusted to point past the object header,
     for example.

     I've changed this to happen in a much more reasonable and
     obvious way: there are now explict macros in AbsCSyn to do the
     adjustment, and the code generator makes calls to these as
     necessary.  Slightly less hackery is necessary in the NCG as
     a result.