Feature: Add all() helper to await successful fulfillment of all operations (#8 by @clue)

// new: limit concurrency while awaiting all operations to complete
$promise = Queue::all(3, $urls, function ($url) use ($browser) {
    return $browser->get($url);
});

$promise->then(function (array $responses) {
    echo 'All ' . count($responses) . ' successful!' . PHP_EOL;
});

Fix: Implement cancellation forwarding for previously queued operations (#7 by @clue)