Replace contexts with resources

- Implements a new macro to generate code for a new structure: TypeMap
- TypeMaps are wrappers for HashMaps that use TypeIDs as keys.
- Refactor the entire codebase to use the new resource structures.
- This commit is the first step towards getting rid of "god context objects everywhere".

engine/src/system.rs

@@ -6,6 +6,7 @@ use std::any::TypeId;
 use std::cell::RefCell;
 use std::rc::Rc;
 use crate::input::InputState;
+use crate::resources::EngineResources;
 
 pub struct SystemContext<'a> {
     pub scene: &'a mut Scene,
@@ -18,10 +19,10 @@ pub trait System {
     /// Initialize the system.
     fn initialize(&mut self) {}
     /// Spawn the first entities of the world.
-    fn load_world(&mut self, _ctx: &mut SystemContext) {}
-    fn handle_event(&mut self, _ctx: &mut SystemContext) {}
-    fn fixed_update(&mut self, _ctx: &mut SystemContext) {}
-    fn frame_update(&mut self, _ctx: &mut SystemContext) {}
+    fn load_world(&mut self, res: &mut EngineResources, scene: &mut Scene) {}
+    fn handle_event(&mut self, res: &mut EngineResources, scene: &mut Scene) {}
+    fn fixed_update(&mut self, res: &mut EngineResources, scene: &mut Scene) {}
+    fn frame_update(&mut self, res: &mut EngineResources, scene: &mut Scene) {}
 }
 
 pub struct SystemManager {