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 value = useMutableSource(
          mutableSource,
          getSnapshot,
          defaultSubscribe,
        );
        Scheduler.unstable_yieldValue(value);
        return value;
      }
      function Text({text}) {
        Scheduler.unstable_yieldValue(text);
        return text;
      }
      const root = ReactNoop.createRoot();
      await act(async () => {
        root.render(
          <>
            <Read getSnapshot={getSnapshotA} />
          </>,
        );
      });
      expect(Scheduler).toHaveYielded(['a0']);
      expect(root).toMatchRenderedOutput('a0');
      await act(async () => {
        root.render(
          <>
            <Read getSnapshot={getSnapshotA} />
            <Read getSnapshot={getSnapshotB} />
            <Text text="c" />