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.
}
function updateTransition(
  config: SuspenseConfig | void | null,
): [(() => void) => void, boolean] {
  const [isPending, setPending] = updateState(false);
  const start = updateCallback(startTransition.bind(null, setPending, config), [
    setPending,
    config,
  ]);
  return [start, isPending];
}
function rerenderTransition(
  config: SuspenseConfig | void | null,
): [(() => void) => void, boolean] {
  const [isPending, setPending] = rerenderState(false);
  const start = updateCallback(startTransition.bind(null, setPending, config), [
    setPending,
    config,
  ]);
  return [start, isPending];
}
let isUpdatingOpaqueValueInRenderPhase = false;
export function getIsUpdatingOpaqueValueInRenderPhaseInDEV(): boolean | void {
  if (__DEV__) {
    return isUpdatingOpaqueValueInRenderPhase;
  }
}
function warnOnOpaqueIdentifierAccessInDEV(fiber) {
  if (__DEV__) {