Re: [streams] Merge ReadableByteStream into ReadableStream (#430)

OK, I have an idea. Maybe we talked about it before. What if we add some kind of `defaultChunkSize` or `defaultReaderChunkSize` option, which would allow the elimination of the `if` branching we see in both examples. So instead of


```js
        if (controller.byobRequest) {
          const v = controller.byobRequest.view;

          return fs.read(fd, v.buffer, v.byteOffset, v.byteLength, position).then(bytesRead => {
            if (bytesRead === 0) {
              return fs.close(fd).then(() => controller.close());
            } else {
              position += bytesRead;
              controller.byobRequest.respond(bytesRead);
            }
          });
        } else {
          // The consumer is using the default reader.
          const buffer = new ArrayBuffer(DEFAULT_CHUNK_SIZE);
          return fs.read(fd, buffer, 0, DEFAULT_CHUNK_SIZE, position).then(bytesRead => {
            if (bytesRead === 0) {
              return fs.close(fd).then(() => controller.close());
            } else {
              position += bytesRead;
              controller.enqueue(new Uint8Array(buffer, 0, bytesRead));
            }
          });
        }
```

we have the `byobRequest` always present for streams with `byob: true` and `defaultReaderChunkSize: DEFAULT_CHUNK_SIZE`.

The name doesn't need to be super-short since this is an advanced option used by stream creators. It could even be something like `defaultReaderAutoAllocateChunkSize`.

WDYT?

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https://github.com/whatwg/streams/pull/430#issuecomment-194603771