This patche fixes Trac #9574.

The previous Note [Renaming associated types] in RnTypes appears to me to be wrong;
it confused class and instance declarations.

I have:

 * Treated kind and type variables uniformly. Both must be bound on the LHS
   of an associated type instance.  Eg
       instance C ('KProxy :: KProxy o) where
          type F 'KProxy = NatTr (Proxy :: o -> *)
   is illegal because 'o' is not bound on the LHS of the instance.

 * Moved the Note to RnSource and fixed it up

This improves the error message from T7938. However it made the code in
T6118 incorrect. We had:
  instance SingE (a :: Maybe k) where
    type Demote a = Maybe (Demote (Any :: k))
and that is now rejected, rightly I think.