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.
root.render(<App getSnapshot={getSnapshotB} />);
    });
    expect(root).toMatchRenderedOutput('Updated B');
    await act(async () => {
      mutateB('Another update');
    });
    expect(root).toMatchRenderedOutput('Another update');
  });
  
  it('should clear the update queue when getSnapshot changes with pending lower priority updates', async () => {
    const source = createSource({
      a: 'initial',
      b: 'initial',
    });
    const mutableSource = createMutableSource(source);
    const getSnapshotA = () => source.value.a;
    const getSnapshotB = () => source.value.b;
    function mutateA(newA) {
      source.value = {
        ...source.value,
        a: newA,
      };
    }
    function mutateB(newB) {
      source.value = {
        ...source.value,
        b: newB,
      };
    }
    function App({toggle}) {