Re: [whatwg/streams] ReadableStream should be an async iterable (#778)

Further questions that have come up as I contemplate this all a bit more seriously:

- Should the async iterator's return() (which is called, remember, when you break out of a for-await-of loop via return/break/throw) cancel the reader, or just release the lock? I.e., if you break out of a loop, should we assume you'll never want to continue looping from the current point and we should clean up for you? On the one hand, that seems presumptuous. On the other hand, it's kind of annoying to make people manually acquire a new reader and call `cancel()` if they break out, especially if that break out is due to an unexpected throw.
- If you iterator to the end, should we release the lock for you? This is somewhat of an edge case, and just governs what happens if someone tries to acquire a new iterator or lock after iterating to the end.

I'm leaning toward auto-canceling and auto-releasing, because otherwise, cleanup is quite annoying:

```js
try {
  for await (const chunk of rs) {
    await somethingThatMightReject(chunk);
  }
} finally {
  try {
    // Might throw if the reader is still locked because we finished successfully.
    await rs.cancel();
  } catch {}
}
```

It's starting to feel a bit magic though....

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