This is just a small optimization on top of what is there.
Add state to Func for tracking if any enabled bodies exist which
allows us to propagate it up to the EventHandler::bool() operator.
In turn, when all bodies of an event's Func have been runtime disabled,
the event itself will not be invoked anymore.
Experiments have shown that this allows runtime toggling of new_event()
without performance impact when disabled. This could enable use-cases
where new_packet() handlers are enabled for a split second once in a
while to either dump or sample raw packet data at runtime.
