[4.x] Forward compatibility with upcoming Promise v3

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Marking this as WIP as the tests currently fail with Promise v3. I've marked the affected tests as incomplete for now, but before merging, the root cause should be fixed instead. Solved via reactphp/promise#229.

Builds on top of reactphp/promise#75 and reactphp/promise#213 and reactphp/promise-timer#54
Changes cherry-picked from #47 for 3.x, but also refs cancellation of async() (see #20 and #42)
Refs #43

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I've updated this to show that this is currently only blocked by reactphp/promise#229. The gist is that we suspend fibers when a promise settles which could previously cause some invalid state in internals of promises to break and not execute any other callbacks. This is now addressed upstream for Promise v3 as this is the only version that shows this problem.

As an alternative, I've also looked into addressing this in this component instead, but this essentially requires suspending the fiber outside of the promise callbacks using a future loop tick. This would essentially revert the major improvements we've recently added via #32 and would thus cause regressions for #27.

As such, I believe addressing this in reactphp/promise#229 is the best way moving forward. The resulting patch looks surprisingly straight-forward, but you're looking at several days worth of work on investigating this root cause and coming up with this minimal solution. Enjoy!

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Updated PR now that reactphp/promise#229 has been merged, this is now ready to be shipped

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Rebased to resolve merge conflict now that #55 is in. This also happens to remove the dependency on PromiseTimer, so this no longer refs reactphp/promise-timer#54