Remove Hugs' special treatment of tuples, and instead have them as
just another Tycon.  This is to make interworking with GHC simpler.

Put tuple entries in the Tycon table.  Modify isTycon, isTuple, tupleOf,
mkTuple and whatIs so that client code doesn't see any difference.
Add allocTupleTycon to manufacture tuple Tycon entries as startup.