useReducer's dispatch should return a promise which resolves once its action has been delivered

(This is a spinoff from [this thread](https://github.com/facebook/react/issues/15240).) It's sometimes useful to be able to dispatch an action from within an async function, wait for the action to transform the state, and then use the resulting state to determine possible further async work to do. For this purpose it's possible to define a `useNext` hook which returns a promise of the next value: ```js function useNext(value) { const valueRef = useRef(value); const resolvesRef = useRef([]); useEffect(() => { if (valueRef.current !== value) { for (const resolve of resolvesRef.current) { resolve(value); } resolvesRef.current = []; valueRef.current = value; } }, [value]); return () => new Promise(resolve => { resolvesRef.current.push(resolve); }); } ``` and use it like so: ```js const nextState = useNext(state); useEffect(() => { fetchStuff(state); }, []); async function fetchStuff(state) { dispatch({ type: 'START_LOADING' }); let data = await xhr.post('/api/data'); dispatch({ type: 'RECEIVE_DATA', data }); // get the new state after the action has taken effect state = await nextState(); if (!state.needsMoreData) return; data = await xhr.post('/api/more-data'); dispatch({ type: 'RECEIVE_MORE_DATA', data }); } ``` This is all well and good, but `useNext` has a fundamental limitation: it only resolves promises when the state _changes_... so if dispatching an action resulted in the same state (thus causing `useReducer` to [bail out](https://reactjs.org/docs/hooks-reference.html#bailing-out-of-a-state-update)), our async function would hang waiting for an update that wasn't coming. What we _really_ want here is a way to obtain the state after the last dispatch has taken effect, whether or not it resulted in the state changing. Currently I'm not aware of a foolproof way to implement this in userland (happy to be corrected on this point). But it seems like it could be a very useful feature of `us…