added collision struct to describe a collision between two entities

src/collision.h

@@ -0,0 +1,18 @@
+#ifndef _fencer_collision_h
+#define _fencer_collision_h
+
+#include "shape.h"
+#include <stddef.h>
+
+struct Collision {
+    Shape* other;
+
+    Vector point;
+    Vector normal;
+    Vector velocity;
+
+    size_t edge_left;
+    size_t edge_right;
+};
+
+#endif // !_fencer_collision_h