This patch combines two changes:
  1. As requested by SimonPJ, the redundancy inherent in having
     LastCall bear actual parameters has been removed.  The actual
     parameters are now carried by a separate CopyOut node.
  2. The internal (to zipper) representation of calls has changed;
     the representation of calling conventions is more orthogonal,
     and there is now no such thing as a 'safe' or 'final' call
     to a CallishMachOp.   This change has affected the interface
     to MkZipCfgCmm, which now provides a static guarantee.  Simon's
     new upstream code will be affected; I've patched the existing
     code in CmmCvt (which becomes ever hairier).