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.
const mutableSource = createMutableSource(source);
    act(() => {
      ReactNoop.render(
        <>
          <Component
            label="a"
            getSnapshot={defaultGetSnapshot}
            mutableSource={mutableSource}
            subscribe={defaultSubscribe}
          />
          <Component
            label="b"
            getSnapshot={defaultGetSnapshot}
            mutableSource={mutableSource}
            subscribe={defaultSubscribe}
          />
        </>,
        () => Scheduler.unstable_yieldValue('Sync effect'),
      );
      expect(Scheduler).toFlushAndYield(['a:one', 'b:one', 'Sync effect']);
      ReactNoop.flushPassiveEffects();
      
      
      Scheduler.unstable_runWithPriority(Scheduler.unstable_LowPriority, () => {
        source.value = 'two';
        expect(Scheduler).toFlushAndYieldThrough(['a:two']);
      });
      const newGetSnapshot = s => 'new:' + defaultGetSnapshot(s);