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.
() => {
          ReactNoop.render(
            <>
              <Component
                label="a"
                getSnapshot={newGetSnapshot}
                mutableSource={mutableSource}
                subscribe={defaultSubscribe}
              />
              <Component
                label="b"
                getSnapshot={newGetSnapshot}
                mutableSource={mutableSource}
                subscribe={defaultSubscribe}
              />
            </>,
            () => Scheduler.unstable_yieldValue('Sync effect'),
          );
        },
      );
      expect(Scheduler).toFlushAndYieldThrough([
        'a:new:two',
        'b:new:two',
        'Sync effect',
      ]);
    });
  });
  
  it('should throw and restart render if source and snapshot are unavailable during a sync update', () => {
    const source = createSource('one');