overflow

[Stephen Weeks]

I finished putting in overflow exceptions today.  The main changes were to the
CPS and Machine ILs.  In both ILs primitive applications now have an additional
PrimInfo field that looks like

	 datatype t =
	    None
	  | Overflow of Label.t

The label is where to jump to if the primitive overflows.  In CPS it is a
nullary continuation.  In Machine, it is just a label to jump to.

Matthew, next time you're at a stable point, we should try to do a merge, and
then you can add in overflow to the X86 backend.  I think it will be easy.