Add monadic IORef functions (e.g. atomicModifyIORefM)

[Gurkenglas]

atomicModifyIORef :: IORef a -> (a -> (a, b)) -> IO b into atomicModifyIORefM :: MonadIO m => IORef a -> (a -> m (b, a)) -> m b and similarly for nearby functions.

runIORefStateT :: MonadIO m => IORef s -> StateT s m b -> m b or perhaps flipped would put the full power of Control.Monad.Trans.State behind this.

Not sure how to prefix this, when base should not have an mtl dependency... but at the very least, IORef a -> (a -> IO (b, a)) -> IO b is overdue.