[streams] Define "tee"ing a stream (#271)

See https://github.com/yutakahirano/fetch-with-streams/issues/14. Fetch has `req.clone()` and `res.clone()` which are currently ill-defined. They should be defined here, in a generic fashion, and probably there should be a user-exposed API for it (one way or another).

Our current experiments with this are in TeeStream.md, but those are probably very outdated.

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/streams/issues/271