Ref: https://github.com/reactjs/rfcs/pull/147#issuecomment-623740218

React version: 0.0.0-experimental-e5d06e34b

## Steps To Reproduce

1. createMutableSource that contains functions
2. getSnapshot returns one of the functions

Link to code example:

https://codesandbox.io/s/jovial-dew-1eg4t?file=/src/App.js

## The current behavior

If getSnapshot returns a function, it will run the function in render. This results in various errors.

My workaround: return an object wrapping a function, and destruct on caller.

> For now, you can work around this by returning a function that returns your function

I don't know how this works as workaround.

## The expected behavior

It would be nice if returning a function just works.

Note: this is not a rare use case in my library https://github.com/dai-shi/use-context-selector
because we often pass `[state, setState]` in a context value.
}
    current.baseQueue = baseQueue = pendingQueue;
    queue.pending = null;
  }
  if (baseQueue !== null) {
    
    const first = baseQueue.next;
    let newState = current.baseState;
    let newBaseState = null;
    let newBaseQueueFirst = null;
    let newBaseQueueLast = null;
    let update = first;
    do {
      const updateExpirationTime = update.expirationTime;
      if (updateExpirationTime < renderExpirationTime) {
        
        
        
        const clone: Update<S, A> = {
          expirationTime: update.expirationTime,
          suspenseConfig: update.suspenseConfig,
          action: update.action,
          eagerReducer: update.eagerReducer,
          eagerState: update.eagerState,
          next: (null: any),
        };
        if (newBaseQueueLast === null) {
          newBaseQueueFirst = newBaseQueueLast = clone;
          newBaseState = newState;
        } else {