GetState interface

[AngusMorton]

A GetState interface would allow us to hide the Store type when using it in situations that shouldn't have access to the Store methods.

This is relevant for cases where you don't want the consumer to be able to do operations on the Store. e.g. redux-observable where the Epic should not have access to the Store because that would allow the Epic to dispatch actions manually, which would be an error.

It would probably look something like:

interface GetState<S> {
   S getState();
}

The Store<S> would implement the GetState interface.