Consider exposing root reducer on store

[lastmjs]

Right now there is no official way to get to the generated root reducer when using createObjectStore. We can provide a custom reducer that is combined with the root reducer, but we can't get to the root reducer itself. My use case for this is performing tasks after the state has been updated, for example persisting the entire state to local storage. I can do it in a listener, but it might be nice to create reducer that encapsulates this functionality, a pattern I've used elsewhere