add Exists API call to check if entity exists

src/EntityComponentReader.cs

@@ -51,4 +51,9 @@ public abstract class EntityComponentReader
 	{
 		return ref ComponentDepot.GetEntity<TComponent>();
 	}
+
+	protected bool Exists(in Entity entity)
+	{
+		return EntityStorage.Exists(entity);
+	}
 }

src/EntityStorage.cs

@@ -9,6 +9,11 @@ internal class EntityStorage
 		return new Entity(idStorage.NextID());
 	}
 
+	public bool Exists(in Entity entity)
+	{
+		return idStorage.Taken(entity.ID);
+	}
+
 	public void Destroy(in Entity entity)
 	{
 		idStorage.Release(entity.ID);

src/IDStorage.cs

@@ -20,6 +20,11 @@ internal class IDStorage
 		}
 	}
 
+	public bool Taken(int id)
+	{
+		return !availableIDs.Contains(id) && id < nextID;
+	}
+
 	public void Release(int id)
 	{
 		availableIDs.Push(id);