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 getSnapshot = React.useCallback(
        ({value}) => Array.from(value.friends),
        [],
      );
      const friends = useMutableSource(
        mutableSource,
        getSnapshot,
        defaultSubscribe,
      );
      return (
        <ul>
          {friends.map(friend => (
            <Friend key={friend.id} id={friend.id} />
          ))}
        </ul>
      );
    }
    function Friend({id}) {
      const getSnapshot = React.useCallback(
        ({value}) => {
          
          
          return value.friends.find(friend => friend.id === id).name;
        },
        [id],
      );
      const name = useMutableSource(