[whatwg/streams] Brainstorming a buffer-reusing version of writable streams (#495)

I'd like to use this meta-issue to track any ideas for a version of writable streams that has some sort of buffer-reuse semantics, like our BYOB readable streams.

#465 has some previous discussion.

One thing I noticed while writing an example is that it's possible for an underlying sink to always transfer away a buffer. This is probably what most high-performance binary data underlying sinks will do, actually. (It's either transfer, copy, or expose raciness to the web.) But this has two drawbacks:

- The underlying sink doesn't see the buffer until the other buffers in the queue ahead of it have been consumed. So it doesn't get detached immediately. So there is a bit of an async race (not a multithreaded one) between any code that manipulates the buffer, and the underlying sink.
- The underlying sink has no opportunity to give the buffer back to the producer if they want to reuse it.

Here is a real example of this. Assume `writableStream` is a stream whose underlying sink transfers any chunks given to it (and errors if not given Uint8Arrays). Then consider this code:

```js
  async function writeRandomBytesForever(writableStream) {
    const writer = writableStream.getWriter();

    while (true) {
      if (Math.random() > 0.5) {
        await writer.ready;
      }

      const bytes = new Uint8Array(1024);
      window.crypto.getRandomValues(bytes);

      const promise = writer.write(bytes);
      doStuffWith(bytes);
      await promise;
    }
  }
```

Here the two problems are illustrated:

- `doStuffWith(bytes)` can maybe modify bytes, or maybe not. (If we did `await writer.ready`, the queue will possibly be empty, in which case you cannot modify, since the underlying sink already transferred it. But if we did not, then you can modify.)
- We have to reallocate new typed arrays every time through the loop; we cannot reuse the same one.

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