Fast Integers.  The rep. of Integers is now

	data Integer = S# Int#
		     | J# Int# ByteArray#

- several new primops added for overflow-detecting arithmetic
- negateInteger# removed; it can be done directly
- integer_0, integer_1 etc. removed.
- the compiler now uses S# where it previously used int2Integer.
- the compiler generates small integers for -2^32 .. 2^32-1, instead
  of -2^29 .. -2^29-1.

- PrelST.State datatype moved to LazyST (its only use).
- some library code (in Time.lhs) still needs cleaning up, it depends
  on the Integer rep.