mtl-2.2: CHANGELOG.markdown

2.2
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* `transformers` 0.4 support
* Added instances for `ExceptT`
* Added `modify'` to `Control.Monad.State.*`

2.1.3.1
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* Avoid importing `Control.Monad.Instances` on GHC 7.8 to build without deprecation warnings.

2.1.3
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* Removed the now-irrelevant `Error` constraint from the `MonadError` instance for `Either e`.