[streams] consider allowing a stream to be removed from a pipeline (#307)

Talking with @trevnorris, it sounds like it would be useful to allow a stream to be removed from a pipeline as an optimization.

For example, if `a.pipeThrough(b).pipeTo(c)`, remove `b` at some point so `a` goes straight to `c`.

This optimization is particularly useful if `a` and `c` are native C++ implementations and removing `b` means its not necessary to execute javascript any more.

Potential APIs for this would be `b.collapse()` or perhaps `b` could return a special value from its write or read methods signalling it should be removed from the pipe chain.

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/streams/issues/307