Yay! One callback to rule all the timers just like it happens for streams! :-)

I'm sure we can use this approach for `LibEventLoop` too, where we currently use one wrapper callback per timer instance, thus avoiding the `&$callback` trick to keep it alive. I just noticed that the 3rd parameter of `event_timer_set()` can be used to pass an argument that gets passed to the callback when called (just like the 5th argument of `Event::__construct()`), the only difference with this loop is that we need to access the internal timers storage to reschedule periodic timers (not provided by pecl/libevent as far as I understand) so we'd really need to pass `[$timers, $timer]` there.