Implement Request/Response `.bytes()` method

[bakkot]

See whatwg/fetch#1732.

Tests at web-platform-tests/wpt#46198.

The above spec PR adds a bytes() method to get a Uint8Array to Body, instead of requiring users to get an ArrayBuffer with arrayBuffer() and then wrap it.

There are also PRs to do the same for Blob and PushMessageData, which copy relevant parts of the Body interface:

• Add bytes() method to Blob for reading bytes into a Uint8Array w3c/FileAPI#197
• Add bytes() method for reading bytes into a Uint8Array w3c/push-api#369