Improve async() by making its promises cancellable

[WyriHaximus]

Since async() returns a promise and those are normally cancelable, implementing this puts them in line with the rest of our ecosystem. As such the following example will throw a timeout exception from the canceled sleep() call.

$promise = async(staticfunction (): int {
echo'a';
await(sleep(2));
echo'b';
returntime();
})();
$promise->cancel();
await($promise);

This builds on top of #15, #18, #19, #26, #28, #30, and #32.

[azjezz]

I think this following case would fail 🤔

$promise = async(function (): int {
echo'a'; await(async(function(): void {
await(async(function(): void {
await(sleep(2)); })());
})());
echo'b';
returntime();
})();

[SimonFrings]

@WyriHaximus looks good to me, only thing I just noticed is that the description for cancellation is missing inside the functions.php.

[WyriHaximus]

@WyriHaximus looks good to me, only thing I just noticed is that the description for cancellation is missing inside the functions.php.

@SimonFrings done 👍

[clue]

@WyriHaximus Some great progress in here, love this feature and how beautifully it aligns with our existing cancellation logic (#9 / #13)!

Added a number of smaller remarks below, otherwise looks like an excellent addition! Semantics make a lot of sense to me, so I'd love to get this in as is, but I do see some room for follow-up discussions once we address reactphp/promise#56 in the future 👍

[WyriHaximus]

@clue Yes this one is the missing link we need to make this package fully fit into our ecosystem. As mentioned on some of your comments, I will create a follow up PR to utilize WeakMap and WeakReference where possible to put the garbage collector in charge of cleaning up promise <-> fiber references. But for the sake of landing this feature, and requiring more time to fully test the behavior there I didn't update this PR with that.

[clue]

@WyriHaximus Some good progress! I agree the WeakMap implementation can be left up to a future PR, only added a couple of remarks below, otherwise LGTM!

[clue]

@WyriHaximus Thanks for the update, changes LGTM, so let's get this shipped!