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.
requestUpdateLane,
  requestEventTime,
  warnIfNotCurrentlyActingEffectsInDEV,
  warnIfNotCurrentlyActingUpdatesInDev,
  warnIfNotScopedWithMatchingAct,
  markRenderEventTimeAndConfig,
  markSkippedUpdateLanes,
} from './ReactFiberWorkLoop.new';
import invariant from 'shared/invariant';
import getComponentName from 'shared/getComponentName';
import is from 'shared/objectIs';
import {markWorkInProgressReceivedUpdate} from './ReactFiberBeginWork.new';
import {requestCurrentSuspenseConfig} from './ReactFiberSuspenseConfig';
import {
  UserBlockingPriority,
  NormalPriority,
  runWithPriority,
  getCurrentPriorityLevel,
} from './SchedulerWithReactIntegration.new';
import {getIsHydrating} from './ReactFiberHydrationContext.new';
import {
  makeClientId,
  makeClientIdInDEV,
  makeOpaqueHydratingObject,
} from './ReactFiberHostConfig';
import {
  getWorkInProgressVersion,
  markSourceAsDirty,
  setWorkInProgressVersion,
  warnAboutMultipleRenderersDEV,