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)