Re: [streams] Add ReadableByteStream observing API (#362)

@domenic 

> I don't quite understand the advantage of these types of solutions over e.g. adding bytesRead to the reader?

The main motivation of this proposal is to satisfy (2) of https://github.com/yutakahirano/fetch-with-streams/issues/37#issuecomment-107836439 as well as the other important requirements and @wanderview's requirement (no one should be able to clear the "used signal") that he showed in his comment https://github.com/yutakahirano/fetch-with-streams/issues/37#issuecomment-110109499 of objection to @yutakahirano's solution.

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