[streams] Custom tee function (#401)

At TPAC, we discussed the use of streams in fetch.  The problem of having to rely on the default implementation of `tee()` is that this introduces the potential need for an infinite buffer.  If the JS were able to provide a tee implementation that didn't depend on creating a clone, that would be significantly more performant.

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/streams/issues/401