feat(async): add async/tee

[kt3k]

This PR adds the utility (tee) for branching the given async iterable into n copies.

This has been originally developed in https://github.com/kt3k/tee_async_iterable as part of file stream processing tool (which is not published yet).

[ry]

LGTM

[exe-dealer]

This code is very fragile and buggy to be in std. It violates async iterator protocol

.next method should be able to queue concurent calls

import { tee } from 'https://deno.land/[email protected]/async/tee.ts';

async function * gen() {
  yield 1;
  yield 2;
}

const [left, right] = tee(gen());

// sequential calls works fine
console.log([await right.next(), await right.next()]);
// [ { value: 1, done: false }, { value: 2, done: false } ]

// should be able to enqueue concurent .next calls
console.log(await Promise.all([ left.next(), left.next() ]));
// [ { value: 1, done: false }, { value: 1, done: false } ]
// but it returns the same result twice

guys are you serious?

https://github.com/denoland/deno_std/blob/f519c159f23a3b2b2b14aa205c7d1af9ae0aacca/async/tee.ts#L98-L100

.return() should be called on source iterator to terminate iteration when all brunches are .return()'ed