I'm not sure why the code was written the way it was, but it seems we
can do better by letting GHC directly reuse `ReaderT`'s instances as
well as relaxing the constraints from

    instance Monad m       => Functor     (InputT m)
    instance Monad m       => Applicative (InputT m)
    instance Monad m       => Monad       (InputT m)

to

    instance Functor     m => Functor     (InputT m)
    instance Applicative m => Applicative (InputT m)
    instance Monad       m => Monad       (InputT m)

This looks the same on GHC 7.8 (pre-AMP) as well as GHC 7.10 (post-AMP)