Backport promise cancellation support to v1 Promise API

[clue]

This is an independent backport of promise cancellation support for the legacy v1 Promise API. It implements the same API as originally implemented in #20, but does not share a single line of code. Original concept courtesy of @jsor.

The main purpose here is backwards compatibility for promise consumers, which can rely on the cancellation API to be available for newer v2 as well as legacy v1 promises.

[jsor]

👍

[clue]

This now also contains the fix from #47 so that the cancellation handler will not be called more than once.

Ready for review

[jsor]

For consistency, you should check here for is_callabe($canceller) and trigger a notice when false like done here, here and here.

[jsor]

Alternatively, you could throw in the constructor, similar to here.

[clue]

Thanks for the hint @jsor, makes perfect sense 👍 I've just pushed an update, I've decided to throw an InvalidArgumentException to be more in line with how the Promise rejects invalid constructors arguments.