Hello everyone!

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:

https://github.com/ekmett/profunctors/pull/70#discussion_r267648958

Thanks,

Dmitrii Kovanikov