change entities marked for destroy list to a hashset

encompass-cs/EntityManager.cs

@@ -7,7 +7,7 @@ namespace Encompass
     {
         private readonly Dictionary<Guid, Entity> IDToEntity = new Dictionary<Guid, Entity>();
 
-        private readonly List<Guid> entitiesMarkedForDestroy = new List<Guid>();
+        private readonly HashSet<Guid> entitiesMarkedForDestroy = new HashSet<Guid>();
 
         private readonly Dictionary<Type, HashSet<IEntityTracker>> componentTypeToEntityTrackers = new Dictionary<Type, HashSet<IEntityTracker>>();
         private readonly Dictionary<Guid, HashSet<IEntityTracker>> entityToEntityTrackers = new Dictionary<Guid, HashSet<IEntityTracker>>();