Use Safe instead of Trustworthy

As of version 0.4.3, [`transformer-base`](http://hackage.haskell.org/package/transformers-base) uses the Safe Haskell extension, so it's possible to switch from Trustworthy to Safe in `monad-control`. This requires adding an additional dependency on the [`stm`](http://hackage.haskell.org/package/stm) package, which is already required by `transformer-base` 0.4.3.