Clean up the AbsC -> AbsC translation of array operations.

* MachOps MO_ReadOSBI and MO_WriteOSBI, which previously did
  array indexing, are gone.  We translate now just to plain
  memory references and explicit address computations.  This
  has the happy side effect that all MachOps now return exactly
  one result (previously it was 0 or 1), cleaning up various
  bits of code.

  As a result the Abstract C structure now contains an unneccessary
  restriction, which is that the result of a MachOp can only be
  assigned to a temporary.  This made sense when MachOps had variable
  numbers of results (0, 1 or 2, originally), but is no longer needed.
  MachOps applied to args could now be allowed to appear as
  arbitrary nodes in expression trees, but so far they are not.

* Get rid of CAddrMode constructor CMem, since it is a special case of
  CVal with a RegRelative of CIndex.

AbstractC is inconsistent and non-orthogonal.  The StixStmt + StixExpr
combination expresses a large part of what AbstractC does in a cleaner
and simpler way, IMO.