I would like to propose to add a `Functor` instance to the `Kleisli` data type from the `Control.Arrow` module. The instance can look like this:
instance Functor m => Functor (Kleisli m a) where
fmap :: (b -> c) -> Kleisli m a b -> Kleisli m a c
fmap f (Kleisli h) = Kleisli (fmap f . h)
{-# INLINE fmap #-}
(<$) :: c -> Kleisli m a b -> Kleisli m a c
c <$ Kleisli h = Kleisli (\a -> c <$ h a)
{-# INLINE (<$) #-}
Having this instance would be really helpful in improving the `profunctors` package by adding QuantifiedConstraints to it. See more details in the discussion below: