Refactor TyCon to eliminate TupleTyCon
This makes TupleTyCon into an ordinary AlgTyCon, distinguished by its AlgTyConRhs, rather than a separate constructor of TyCon. It is preparatory work for making constraint tuples into classes, for which the ConstraintTuple tuples will have a TyConParent of a ClassTyCon. Tuples didn't have this possiblity before. The patch affects other modules because I eliminated the unsatisfactory partial functions tupleTyConBoxity and tupleTyConSort. And tupleTyConArity which is just tyConArity.