Fix Trac #3265: type operators in type/class declarations
We should accept these: data a :*: b = .... or data (:*:) a b = ... only if -XTypeOperators is in force. And similarly class decls. This patch fixes the problem. It uses the slightly-nasty OccName.isSymOcc, but the only way to avoid that is to cach the result in OccNames which seems overkill to us.