[fetch] Developer-controlled streams for request/response (#88)

First https://github.com/yutakahirano/fetch-with-streams needs to be integrated. So developers can access the stream of request/response. Next is giving them control over the stream.

* What if you do `fetch(url, { body })` where body is a stream which gets random stuff (not bytes) enqueued in it?
* Result: when fetch reads from body, it notices a non-`BufferSource`, and ... does what?
 * Returned promise rejects
 * Cancel the stream
 * On the wire, it just terminates somehow? How does HTTP work?
* Require "cors-with-forced-preflight" (you need a preflight in most cases anyway, and it's also required for progress events, which streams enable) (in particular you need it for everything that isn't a POST with one of the `<form>` MIME types)
* Always do content-encoding: chunked, and do uploads as chunked encoding, which we need to spec in some way

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/fetch/issues/88