Word{8,32}.neg

[Stephen Weeks]

> I  would  MOST  STRONGLY  like  to  agree  that  Word8.~  and Word32.~ should
> DEFINITELY exist.  I think that at some point in the  past  I  sent  mail  to
> Reppy  about  this but didn't get an answer.  This is just confusion on their
> part.  It is a completely reasonable operation and should  definitely  exist.
> The fact that it could be used in the optimization is just another indication
> of the fact that this is a basic operation.

OK.  I added them to MLton.Word and MLton.Word8.