This change significantly reduces the amount of work done by event handlers. Instead of traversing all of the priorities and event parent chain multiple times, we reduce event handlers down to a simple list, which doesn't require any logic to iterate over.
Previously, calling an event with lots of parents costed more than an event which directly descended from Event.
In addition, we had to do a lot of usually useless work to check all priorities, when in practice, only NORMAL will be used in almost all cases.
This change makes it more cost effective to implement the feature suggested by #5678; however, it will still require additional changes.
