Add the possibility to abort or cancel a promise

[king-of-poppk]

I guess it makes sense for future-backed promises, not sure in general.

• See WISH: Add method for interrupting / terminating a future futureverse/future#93.

[HenrikBengtsson]

Update from the future author: The next release of the future package will support interrupt():ing a future. This should open up the door for promises to interrupt future promises too, e.g. detect when the user requests an interrupts and then call interrupt() on the relevant futures.

[schloerke]

Marking as not implemented as the promises package does not have a native cancel method.

To add this would not be trivial work. I'm marking as not planned.

[schloerke]

I mis-read the issue. Reopening as there could be a cancel.promise() method.

But I'm not hopeful on any timeline for an implementation.