* I found a bug in 'generalize' in TcTyClsDecls.kcTyClGroup, where
  the kind variables weren't being turned into proper TyVars, so
  we got (skolem) TcTyVars in TyCons, which shouldn't happen.  Fix
  was easy.

* Similarly TcHsType.kindGeneralizeType wasn't turning the forall'd
  TcTyVars into TyVars.  To achieve this I defined TcHsTyn.zonkSigType.

* All this allowed me to remove awkward and ill-explained bit of
  footwork on DFunIds in Inst.newClsInst

This is just refactoring, but it does make the printout from
-ddump-deriv make a bit more sense by not grautuitiously cloning
type variables.  In the display I was seeing

   instance C [a_df4] where
      f x = ...a_dx5...

where actually the d_df4 and a_dx5 were the same.