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.