Structured cloning/transfering of promises (and streams)

[Domenic Denicola]

FYI, I've started working out how structured cloning could work for promises: https://github.com/dslomov-chromium/ecmascript-structured-clone/issues/6

This is part of a larger initiative to figure out the story for both structured cloning and tranferability for both promises and the upcoming I/O streams under development: https://github.com/dslomov-chromium/ecmascript-structured-clone/issues/5. That particular issue is the first step.

In turn, this is part of a larger initiative discussed on-list in the past to bring structured cloning and transferability into ECMAScript, instead of having it defined in HTML: https://github.com/dslomov-chromium/ecmascript-structured-clone

Please feel free to chime in on those issues; would love some feedback to make sure I'm on the right track, especially from implementers.