* Refactor CLabel.RtsLabel to CLabel.CmmLabel

* Refactor CLabel.RtsLabel to CLabel.CmmLabel

The type of the CmmLabel ctor is now
  CmmLabel :: PackageId -> FastString -> CmmLabelInfo -> CLabel

 - When you construct a CmmLabel you have to explicitly say what
   package it is in. Many of these will just use rtsPackageId, but
   I've left it this way to remind people not to pretend labels are
   in the RTS package when they're not.

 - When parsing a Cmm file, labels that are not defined in the
   current file are assumed to be in the RTS package.

   Labels imported like
      import label
   are assumed to be in a generic "foreign" package, which is different
   from the current one.

   Labels imported like
      import "package-name" label
   are marked as coming from the named package.

   This last one is needed for the integer-gmp library as we want to
   refer to labels that are not in the same compilation unit, but
   are in the same non-rts package.

   This should help remove the nasty #ifdef __PIC__ stuff from
   integer-gmp/cbits/gmp-wrappers.cmm