Disambiguate fetch-request-streams feature

[jgraham]

Rewrite the fetch-request-streams feature to make it more clear what precise property it covers, and add a new fetch-body-stream feature to cover the body option to the fetch function supporting ReadableStream objects.

[jgraham]

Based on web-platform-tests/interop#1072 (comment) another option here is to just move api.Request.Request.request_body_readablestream into Fetch upload streams, although it won't accurately reflect the fact that some Request.body usecases already work in Safari (but not Firefox).

[jgraham]

OK, I've updated this to just make the more minimal change of requiring that the Request body property feature just requires both that you get a body property on Request and also that you can specify it in the constructor.

I'm not delighted with that, because it obscures the fact that part of the feature is already available in Safari, but it's better than the status quo.

[captainbrosset]

The only thing I'm not 100% sure about is the name. Isn't it a problem that we're loosing the "stream" part of it?
According to MDN, the body part of a request can either be null or a stream.
So, really, it's only used for streaming requests to a server, right?

What other things can it be used for?