change the zipper representation of calls
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).