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.
};
let HooksDispatcherOnMountInDEV: Dispatcher | null = null;
let HooksDispatcherOnMountWithHookTypesInDEV: Dispatcher | null = null;
let HooksDispatcherOnUpdateInDEV: Dispatcher | null = null;
let HooksDispatcherOnRerenderInDEV: Dispatcher | null = null;
let InvalidNestedHooksDispatcherOnMountInDEV: Dispatcher | null = null;
let InvalidNestedHooksDispatcherOnUpdateInDEV: Dispatcher | null = null;
let InvalidNestedHooksDispatcherOnRerenderInDEV: Dispatcher | null = null;
if (__DEV__) {
  const warnInvalidContextAccess = () => {
    console.error(
      'Context can only be read while React is rendering. ' +
        'In classes, you can read it in the render method or getDerivedStateFromProps. ' +
        'In function components, you can read it directly in the function body, but not ' +
        'inside Hooks like useReducer() or useMemo().',
    );
  };
  const warnInvalidHookAccess = () => {
    console.error(
      'Do not call Hooks inside useEffect(...), useMemo(...), or other built-in Hooks. ' +
        'You can only call Hooks at the top level of your React function. ' +
        'For more information, see ' +
        'https:fb.me/rules-of-hooks',
    );
  };
  HooksDispatcherOnMountInDEV = {
    readContext<T>(
      context: ReactContext<T>,
      observedBits: void | number | boolean,
    ): T {