Give MonadTrans a QuantifiedConstraints superclass

I made a patch for transformers (issue) adding a superclass to MonadTrans. This is the posterboy of the QuantifiedConstraints extension; indicating that MonadTrans trans lifts Monad m to Monad (trans m).

class (forall m. Monad m => Monad (t m)) => MonadTrans t where
  lift :: Monad m => m a -> t m a

I haven't contributed using darcs before so I am making a ticket here as well to attract discussion and making sure I'm doing everything right.