[Feature] Move to ECS driven model

[kanerogers]

Background

• It should be possible to write a game by only writing ECS code.
• The interface between your game and the engine is world. That's it.
• Each context should have a fn reconcile(self, world: &mut World) method that reconciles any deltas between that context and the World.
  • wrinkle: This could be implemented as a trait, but since our Contexts are known statically at runtime, there's probably no point. Though, perhaps users could someday provide their own contexts, or disable ones they don't need?

TODO

• [Feature] Make PhysicsContext ECS driven #379
• [Feature] Make InputContext ECS driven #380
• [Feature] Make asset_importer ECS driven #381
• [Feature] Make AudioContext entirely ECS driven #382