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.
jest.resetModules();
  jest.useFakeTimers();
  ReactFeatureFlags = require('shared/ReactFeatureFlags');
  ReactFeatureFlags.enableSchedulerTracing = true;
  ReactFeatureFlags.enableProfilerTimer = true;
  React = require('react');
  ReactNoop = require('react-noop-renderer');
  Scheduler = require('scheduler');
  useMutableSource = React.useMutableSource;
  act = ReactNoop.act;
}
describe('useMutableSource', () => {
  const defaultGetSnapshot = source => source.value;
  const defaultSubscribe = (source, callback) => source.subscribe(callback);
  function createComplexSource(initialValueA, initialValueB) {