Add polymorphic improve variant

[treeowl]

improve :: Functor f => (forall m. MonadFree f m => m a) -> Free f a
improve m = lowerCodensity m

lowerCodensity :: Monad f => Codensity f a -> f a

The basic idea of improve is that the passed action does not depend on the choice of free monad construction (I don't actually remember why this matters, but that's the idea). The specific choice of Free f as the result monad, however, seems to be arbitrary. Thus

improve' :: (Functor f, MonadFree f m') =>
            (forall m . MonadFree f m => m a) -> m' a
improve' m = lowerCodensity m

[treeowl]

Unrelated: why improve m = lowerCodensity m and not just improve = lowerCodensity?