Add IdentifierRef.Count to get count in-use

src/gaemstone.ECS/IdentifierRef.cs

@@ -14,7 +14,9 @@ public unsafe class IdentifierRef
 	public bool IsPair     => Id.IsPair;
 	public bool IsWildcard => Id.IsWildcard;
 	public bool IsValid    => ecs_id_is_valid(World, this);
-	public bool IsInUse    => ecs_id_in_use(World, this);
+
+	public bool IsInUse => ecs_id_in_use(World, this);
+	public int  Count   => ecs_count_id(World, this);
 
 	public IdentifierRef(World world, Identifier id)
 		{ World = world; Id = id; }