Log Message:
Update to mtl-2.2.2

* Control.Monad.Identity now re-exports Control.Monad.Trans.Identity
* Fix a bug in which Control.Monad.State.Class.modify' was not as
  strict in the new state as its counterparts in transformers
* Add a MonadError () Maybe instance
* Add liftEither :: MonadError e m => Either e a -> m a to
  Control.Monad.Except{.Class}
* Add a MonadWriter w ((,) w) instance (when built against base-4.9 or
  later)