Add a spec for an extended API for binary data handling

I think we should add a step here validating that `arrayBuffer` is a real ArrayBuffer. That seems important if we're to guarantee that implementations have the ability to do direct memory copies or backing the AB with the appropriate kernel-space buffers.

To do this, see e.g. https://people.mozilla.org/~jorendorff/es6-draft.html#sec-get-arraybuffer.prototype.bytelength. You need four checks, plus another line to extract the [[ArrayBufferByteLength]] internal slot. I will file a bug on ES to give us a EnsureIsArrayBuffer(ab) operation we can use to obviate the four checks but in the meantime probably just want to inline them.