utils bundle and use update atom

[dai-shi]

ref: #26

This adds new utils bundle, that is separated from core.

import { atom, useAtom } from 'jotai'
import { useUpdateAtom } from 'jotai/utils'

const countAtom = atom(0)

const Counter = () => {
  const [count] = useAtom(countAtom)
  return <div>count: {count}</div>
}

const Controls = () => {
  const setCount = useUpdateAtom(countAtom)
  const inc = () => setCount(c => c + 1)
  return <button onClick={inc}>+1</button>
}

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[dai-shi]

https://codesandbox.io/s/react-typescript-forked-3q11k
Working as expected.