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.
refs.setSnapshot = setSnapshot;
    
    const maybeNewVersion = getVersion(source._source);
    if (!is(version, maybeNewVersion)) {
      const maybeNewSnapshot = getSnapshot(source._source);
      if (!is(snapshot, maybeNewSnapshot)) {
        setSnapshot(maybeNewSnapshot);
        const currentTime = requestCurrentTimeForUpdate();
        const suspenseConfig = requestCurrentSuspenseConfig();
        const expirationTime = computeExpirationForFiber(
          currentTime,
          fiber,
          suspenseConfig,
        );
        setPendingExpirationTime(root, expirationTime);
      }
      
      
      
      
      
      
      markRootExpiredAtTime(root, getLastPendingExpirationTime(root));
    }
  }, [getSnapshot, source, subscribe]);
  
  dispatcher.useEffect(() => {
    const handleChange = () => {