Added a MonadState instance.

We've had a user `fragramus` on the Haskell channel for the last couple of days armwrestling with transformer stacks because he put `CRandT` on the bottom of the stack, so I figured I'd just fix the issue at the source.

This patch adds a `MonadState` instance to `CRandT`.

You may want to consider also lifting `MonadWriter`, etc. 

The instances are remarkably straightforward.