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 unsubscribeA = jest.fn();
    const subscribeA = jest.fn(s => {
      const unsubscribe = defaultSubscribe(s);
      return () => {
        unsubscribe();
        unsubscribeA();
      };
    });
    const unsubscribeB = jest.fn();
    const subscribeB = jest.fn(s => {
      const unsubscribe = defaultSubscribe(s);
      return () => {
        unsubscribe();
        unsubscribeB();
      };
    });
    act(() => {
      ReactNoop.renderToRootWithID(
        <Component
          label="only"
          getSnapshot={defaultGetSnapshot}
          mutableSource={mutableSource}
          subscribe={subscribeA}
        />,
        'root',
        () => Scheduler.unstable_yieldValue('Sync effect'),
      );
      expect(Scheduler).toFlushAndYield(['only:a-one', 'Sync effect']);
      ReactNoop.flushPassiveEffects();
      expect(source.listenerCount).toBe(1);