Add Marc Van Dongen's Integer improvements.  Specifically:

	- new primops: gcdInt#, gcdIntegerInt#, divExact#,
	  quotInteger#, remInteger#.

	- new definitions of quot and rem for Integer in
	  PrelNum (using the new quotInteger# and remInteger#
	  primops instead of quotRemInteger#).  Should be
	  slightly faster than before.  div & mod aren't
	  likewise optimised (yet).

	- specialisations of gcd for Int and Integer, and
	  lcm for Integer in PrelNum.