Removed the old Memory Arena and converted all to the new one

Game/Entity/EntityManager.cpp

@@ -11,7 +11,16 @@ namespace Game
 
     EntityID EntityManager::ID = 0;
 
-    void InitEntityManager() {}
+    void InitEntityManager(Juliet::NonNullPtr<Juliet::Arena> arena)
+    {
+        Manager.Arena = arena.Get();
+        Manager.Entities.Create(arena);
+    }
+
+    void ShutdownEntityManager()
+    {
+        Manager.Entities.Destroy();
+    }
 
     EntityManager& GetEntityManager()
     {