Proposal: Add Compositor class as superclass of Arrow

[apfelmus]

Cale Gibbard wrote:
>
> class Functor c m => Monad c m where
>    return :: a -> m a
>    join :: m (m a) -> m a
>    (>>=) :: m a -> (c a (m b)) -> m b
>    x >>= f = join (f . x)
>    join x = x >>= id

We may even want

    return :: c () a -> m a

since there doesn't need to be an embedding from (all of) Hask to  c , 
i.e. the domain and image types may well be phantoms.

Regards,
apfelmus