Resources can only wait for type erased events

[MrBurmark]

Currently you have to create a type erased event in order to have a resource wait_for it. Does it make sense to allow resources to wait_for their own event type to avoid this, maybe even wait_for another resource of their own type as syntactic sugar?

[trws]

Yeah, that's a combination of problems, since wait_for should really take the event by value in which case it would just get auto-wrapped and we also don't have a specific version. Maybe we should rip off the bandaid and actually fix this set of issues, along with the getting a non-erased event problem, before they get too much usage?