This makes XHR now rely on Fetch, which allows it to correct send
Origin and Referer headers, CORS-preflight and filtering and many other
goodies.
The main thing that's missing is Streams, which means we can't properly
produce progress events or switch to the Loading ready state.
This also doesn't implement the Document responseType just yet.
Usually operations that mirror AOs from the Infra spec are simply part
of the underlying data structures in AK directly, but these don't seem
generally useful enough to add them as ByteBuffer methods.