feat: Add for await (const contact of room) {} support by ES6 iterators override

[huan]

After support this, we can do:

const room = await Room.find('test room')
for await (const contact of room) {
  console.log('Contact', contact, 'is in room', room)
}

See

• Symbol.iterator will allow you to override the of operator
• for-await-of and synchronous iterables
• [Asynchronous iteration for JavaScript(https://github.com/tc39/proposal-async-iteration)

[huan]

Should have been done by the commit.