Guest world

[philpax]

This is an attempt to switch the guest over to using a World struct instead of global functions to help resolve #725 and #230 and to enable #71 in the future.

However, there are a few blocking issues:

• The AnimationNode Clone and Drop implementations rely on modifying the ECS globally, which doesn't work when there's a World floating around. I've disabled this for now just to make it compile, but that's not a workable solution.
• Anything involving callbacks requires passing the World through, including event subscriptions.
• Every bit of Ambient code will need to be updated.

I'm leaving this as a draft PR to pick up later, but it seems like something we should do at some point.

[FredrikNoren]

@philpax Hm yeah.. I'm not entirely happy with AnimationNode, and it feels like this might actually expose that the design isn't that good. I'll take a look if I can figure out how to do the animation stuff differently

[philpax]

With the recent animation API changes, this is now viable again. I'll revisit once I'm done with other work.