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.
if (!is(newState, hook.memoizedState)) {
      markWorkInProgressReceivedUpdate();
    }
    hook.memoizedState = newState;
    
    
    
    
    if (hook.baseQueue === null) {
      hook.baseState = newState;
    }
    queue.lastRenderedState = newState;
  }
  return [newState, dispatch];
}
type MutableSourceMemoizedState<Source, Snapshot> = {|
  refs: {
    getSnapshot: MutableSourceGetSnapshotFn<Source, Snapshot>,
    setSnapshot: Snapshot => void,
  },
  source: MutableSource<any>,
  subscribe: MutableSourceSubscribeFn<Source, Snapshot>,
|};
function readFromUnsubcribedMutableSource<Source, Snapshot>(
  root: FiberRoot,
  source: MutableSource<Source>,
  getSnapshot: MutableSourceGetSnapshotFn<Source, Snapshot>,
): Snapshot {