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.
};
    newQueue.dispatch = setSnapshot = (dispatchAction.bind(
      null,
      currentlyRenderingFiber,
      newQueue,
    ): any);
    stateHook.queue = newQueue;
    stateHook.baseQueue = null;
    snapshot = readFromUnsubcribedMutableSource(root, source, getSnapshot);
    stateHook.memoizedState = stateHook.baseState = snapshot;
  }
  return snapshot;
}
function mountMutableSource<Source, Snapshot>(
  source: MutableSource<Source>,
  getSnapshot: MutableSourceGetSnapshotFn<Source, Snapshot>,
  subscribe: MutableSourceSubscribeFn<Source, Snapshot>,
): Snapshot {
  const hook = mountWorkInProgressHook();
  hook.memoizedState = ({
    refs: {
      getSnapshot,
      setSnapshot: (null: any),
    },
    source,
    subscribe,
  }: MutableSourceMemoizedState<Source, Snapshot>);
  return useMutableSource(hook, source, getSnapshot, subscribe);
}
function updateMutableSource<Source, Snapshot>(