Add MonadMorph type class

Hello! Is it possible to add a MonadMorph type class to the library? Its implementation is very simple:

```
class MonadMorph m n where
morph :: m a -> n a
```

It allows to define possible morphisms between monads. Right now I and some of my friends are using such class in connection to mmorph lib and I think it should be just built in, what do you think? :)