- add touch# and foreignObjToAddr# primops.

- add these functions to ForeignObj:

	withForeignObj  :: ForeignObj -> (Addr -> IO a) -> IO a
	touchForeignObj :: ForeignObj -> IO ()

- foreignObjToAddr, which was previously implemented using a _casm_, now
  uses the foreignObjToAddr# primop.

- replace implementations of readXXXOffForeignObj and writeXXXOffForeignObj
  using the withForeignObj and the Addr operations.  ForeignObj.lhs now has
  no _casms_ in it!  (it still can't be compiled with the NCG though, because
  it has a f.i.d.)