Re: [fetch] Replace byte stream with ReadableStream. (#200)

This looks really great @yutakahirano. Apart from the above nits, I think whenever we allocate a `Uint8Array` and corresponding buffer, we need to account for that throwing. That could be a follow up if you want since I know we don't do it now either.

I would also greatly appreciate it if @domenic and @wanderview could have a look at this PR (search for `Overview.src.html` to get the diff you want to read).

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