refactor project structure (start of redoing atomic branch without atomics)

core/src/util/mod.rs

@@ -0,0 +1,18 @@
+mod arena;
+mod borrow;
+mod change;
+mod id;
+mod math;
+mod refcount;
+mod vec2;
+
+pub use arena::*;
+pub use borrow::*;
+pub use change::*;
+pub use id::*;
+pub use math::*;
+pub use refcount::*;
+pub use vec2::*;
+
+pub type HashMap<K, V> = fxhash::FxHashMap<K, V>;
+pub type HashSet<K> = fxhash::FxHashSet<K>;