Give helper type families more precise kinds

[RyanGlScott]

When promoting a class or instance method, we generate a "helper" type family definition that contains the actual implementation of the class or instance method. Prior to this patch, it was possible that the kind of the helper type family could be more polymorphic than desired. For instance, singletons-th would promote this:

$(promote [d|
  type MyApplicative :: (Type -> Type) -> Constraint
  class Functor f => MyApplicative f where
    ap :: f (a -> b) -> f a -> f b

    rightSparrow :: f a -> f b -> f b
    rightSparrow x y = ap (id <$ x) y
  |])

To this:

type PMyApplicative :: (Type -> Type) -> Constraint
class PMyApplicative f where
  type Ap (x :: f (a ~> b)) (y :: f a) :: f b
  type RightSparrow (x :: f a) (y :: f b) :: f b
  type RightSparrow x y = RightSparrowDefault x y

-- The helper type family
type RightSparrowDefault :: forall f a b. f a -> f b -> f b
type family RightSparrowDefault x y where
  RightSparrowDefault x y = Ap (IdSym0 <$ x) y

Note that GHC would generalize the standalone kind signature of RightSparrowDefault to:

type RightSparrowDefault :: forall {k} (f :: k -> Type) (a :: k) (b :: k).
                            f a -> f b -> f b

This is more general than intended, as we want f to be of kind Type -> Type instead. After all, we said as much in the standalone kind signature for MyApplicative! This excessive polymorphism doesn't actively cause problems in today's GHC, but they will cause problems in a future version of GHC that implements GHC#23515. (See #601.)

This patch resolves the issue by propagating kind information from the class's standalone kind signature (or, in the case of instance declarations, the instance head) to the helper type family declarations. After this patch, we now generate the following kind for RightSparrowDefault (as verified by the new T601a test case):

type RightSparrowDefault :: forall (f :: Type -> Type) a b.
                            f a -> f b -> f b

This piggybacks on machinery that was added in #596 to do most of the heavy lifting.

Resolves the "Overly polymorphic promoted class defaults" section of #601.