Abstractions for synchronous or asynchronous script calls with responses

[makspll]

I.e. right now it's not very easy to call a script and get something back out of it without digging through the codebase heavilly.

It should be as easy as:

script.call::<Callback>(vec![])

the sync vs async approaches have benefits and drawbacks, doing this async via messages, you could call any script from any system without exclusive world access

[makspll]

This is now addressed via #327, for asynchronouse calls, we have events, future improvements however, should allow us to poll for events within the context of a system, and get an event response