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.

[neongreen]

atomicModifyIORefM :: MonadIO m => IORef a -> (a -> m (b, a)) -> m b

This might be impossible even for atomicModifyIORefM :: IORef a -> (a -> IO (b, a)) -> IO b, or at least it would require adding a new primop to GHC – but I'm not sure. Should ask someone on #ghc about that.

[HuwCampbell]

The IO action m (b, a) could write to or read from the IORef, so it can't be atomic.