improving the synatax and semantics of the privileged import

 Typical use might be:

  import Prelude
  import privileged Prelude ( IORef , unsafePerformIO )

 Which means please ignore the export that comes with Prelude,
 and let me at compiler internal magic operations, IORef and
 unsafePerformIO (both are later exported by IOExt)

I've also updated the stdlib files to use this (hugs only :-).