Re: [streams] Generalized and precise flow/buffer control (#301)

As Yutaka has been advocating, we should stop attempting to specify direct connection between the "underlying source" which the JS code faces and the readable (byte) stream operation. Not directly, but just have a good API surface that **could** be utilized for optimization if the implementor of the readable (byte) stream wants. Bring your own buffer style reader allows the implementor to directly connect `read(view)` with ReadFile, etc, but it's not required. As we discussed offline, the JS library may be requested to drain data from underlying source into an ArrayBuffer in JS given some consuming pressure. In such case, we don't pass the view provided by the consumer to ReadFile, but copies data already read out to the provided view.

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