Impl HandlerParam for &World, &mut World

[andrewgazelka]

Pros

• This is possible to do in Bevy.
• It might be nice given Make World interface more convenient. #57 improvements and until Expose the event bump allocator. #44 is implemented, which would allow non-static events when we do not have access

Cons

• I am not sure how much this would be used. Right now, I have everything that would need a &World or &mut World running outside of a system.

[rj00a]

&World might be doable, but &mut World within a handler would lead to unsoundness. It would be possible to send an event for the currently running handler, causing mutable aliasing. You could also do things like remove handlers which would invalidate the iterator over the current HandlerList.