[streams] Identity transform performance and ArrayBufferView ownership (#323)

(based on @yutakahirano's analysis)

An identify transform can be designed to pass the `ArrayBufferView`s (after detaching) written to the writable side as-is to the readable side. This is good since no copy happens.

We have three choices in implementing this:
- (a) on `.write()`, the transform takes ownership of the `ArrayBufferView` permanently
- (b) on `.write()`, the transform takes ownership but returns it later (notify ownership release by fulfilling the promise returned by `.write()`)
- (c) on `.write()`, the transform reads out all the bytes from the `ArrayBufferView`. It never takes ownership.

(a) allows the transform to pass the `ArrayBufferView` as-is (after detaching) to the consumer through `ReadableStream` without ack capability (non-BYOB), but disallows the producer to reuse the buffer.

(b) allows the producer to reuse the buffer, but if the readable side is `ReadableStream` without ack capability (non-BYOB), there's no way to get the buffer returned by the consumer, so, we need to copy data inside the transform. To eliminate copy, we need `ReadableStream` with ack.

When BYOB reading is used, we anyway need to copy bytes.

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/streams/issues/323