[4.x] Consistent cancellation semantics for async() and coroutine()

[clue]

This changeset ensures we're using consistent cancellation semantics for async() and coroutine(). In particular, calling cancel() on the resulting promise will now try to cancel the first pending operation only. Cancelling a pending operation would usually throw an exception and thus reject the resulting promise. If this exception in caught inside the fiber/coroutine and another operation is started or if the pending operation does not support cancellation, the fiber/coroutine may continue executing.

Refs #42
Changes for coroutine() cherry-picked from #54 for 3.x, but also refs cancellation of async() (see #20 and #34)
Builds on top of #20, #34 and #48