Fix Trac #2334: validity checking for type families
When we deal with a family-instance declaration (TcTyClsDecls.tcFamInstDecl) we must check the TyCon for validity; for example, that a newtype has exactly one field. That is done all-at-once for normal declarations, and had been forgotten altogether for families. I also refactored the interface to tcFamInstDecl1 slightly. A slightly separate matter: if there's an error in family instances (e.g. overlap) we get a confusing error message cascade if we attempt to deal with 'deriving' clauses too; this patch bales out earlier in that case. Another slightly separate matter: standalone deriving for family instances can legitimately have more specific types, just like normal data decls. For example data instance F [a] = ... deriving instance (Eq a, Eq b) => Eq (F [(a,b)]) So tcLookupFamInstExact can a bit more forgiving than it was.