Renamer changes:

 - for a toplevel type signature

	f :: ty

   the name 'f' refers to a local definition of 'f' - i.e., don't
   report 'f' as clashing with any imported 'f's.

 - tidied up the handling of fixity declarations - misplaced fixity
   declarations inside class decls, e.g.,

      class F a where
       infix 9 `f`
       g :: a -> Int

   are now caught and reported as errors. Robustified the renaming
   of class decls.