1. Arrange that w/w records unfoldings
   And that the simplifier preserves them

2. Greatly improve structure of checking user types in the typechecker
   Main changes:
	TcMType.checkValidType checks for a valid type
	TcMonoType.tcHsSigType uses checkValidType
	Type and class decls use TcMonoType.tcHsType (which does not
		check for validity) inside the knot in TcTyClsDecls,
		and then runs TcTyDecls.checkValidTyCon
		or TcClassDcl.checkValidClass to check for validity
		once the knot is tied