[whatwg/streams] Garbage-free streaming fetch into a WebAssembly (Shared)ArrayBuffer? (#1057)

Looking at the BYOB reading example at https://streams.spec.whatwg.org/#example-manual-read-bytes , the code looks awkward because of the large number of JS garbage that is generated. High performance animation-heavy WebAssembly applications strive to operate garbage-free, because JS GC pressure is known to generate microstuttering in WebGL rendering/animation.

Would it be possible to create an API that would enable garbage-free streaming into a(n) (Shared)ArrayBuffer?

The BYOB example on the page looks like

```js
const reader = readableStream.getReader({ mode: "byob" });

let startingAB = new ArrayBuffer(1024);
readInto(startingAB)
  .then(buffer => console.log("The first 1024 bytes:", buffer))
  .catch(e => console.error("Something went wrong!", e));

function readInto(buffer, offset = 0) {
  if (offset === buffer.byteLength) {
    return Promise.resolve(buffer);
  }

  const view = new Uint8Array(buffer, offset, buffer.byteLength - offset); // (* garbage *)
  return reader.read(view).then(newView => { // ( * also garbage? *)
    return readInto(newView.buffer, offset + newView.byteLength);
  });
}
```

The stream avoids extra copy, but generates garbage. Comparing to a non-BYOB variant for a fetch:

```js
function downloadFile(url) {
 return fetch(url)
 .then(response => {
  return [response.body, new Uint8Array(response.headers.get('Content-Length'))];
 }).then(body => {
  var reader = body[0].getReader();
  var buf = body[1];
  var totalDownloaded = 0;
  function onChunkDownloaded(data) {
   if (data.value) {
    buf.set(data.value, totalDownloaded); // (* excess memcpy *)
    totalDownloaded += data.value.length;
   }
   if (data.done) {
    return buf;
   } else {
    return reader.read().then(onChunkDownloaded);
   }
  }
  return reader.read().then(onChunkDownloaded);
 });
}
downloadFile('myLargeFile.dat').then(buf => {
 console.log('Downloaded file:');
 console.dir(buf);
});
```
This code does not generate excessive typed array views, but it has an extra memory copy.

I wonder if there would be a way to get to a JS GC free + zero copy way of downloading files?

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