Re: [streams] First draft at tee algorithms, for critique (#302)

If the consumer of either branch really wants to interact with the original stream via `cancel()`, we should ensure that the consumer's `cancel()` call results in calling `cancel()` on the original stream.

I basically think such needs should be taken care of by more complex custom tee-ing code. But if we're to build some helper for convenience (so that any change on the consumer code is unnecessary), I'd propose something like the following:

- `rs.tee()` takes `cancelPropagator`
- `rs.tee()` calls `cancelPropagator.init(cancel)` where `cancel` is a function that calls `rs.cancel()`
- when the first branch is `cancel()`-ed, `cancelPropagator.handleFirst(reason)` is called where `reason` is the argument the consumer of the first branch passed to `cancel()` call. The return value of `handleFirst` is used for resolving the promise returned to the consumer of the first branch for its `cancel()` call
- the same for the second branch
- if `rs.tee()` is invoked without `cancelPropagator`, the default handler is used which is:

```
class DefaultCancelPropagator {
  constructor() {
    this._firstCancelled = false;
    this._secondCancelled = false;
  }
  init(cancel) {
    this._cancel = cancel;
  }
  handleFirst() {
    this._firstCancelled = true;
    if (this._secondCancelled) {
      this._cancel();
    }
    return undefined;
  }
  handleSecond() {
    this._secondCancelled = true;
    if (this._firstCancelled) {
      this._cancel();
    }
    return undefined;
  }
}
```

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https://github.com/whatwg/streams/pull/302#issuecomment-84816393