This patch allows

     class C a where
       type T a b :: *
     instance C Int
       type T Int b = b -> b

That is, T has a type index 'b' that is not one of the class
variables.

On the way I did a good deal of refactoring (as usual), especially in
TcInstDcls.tcLocalInstDecl1, which checks for consistent instantiation
of the class instance and the type instance.  Less code, more
expressiveness.  See Note [Checking consistent instantiation]