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".

core/src/utils/mod.rs

@@ -0,0 +1,2 @@
+pub mod typemap;
+mod managers;