only build DebugSystem in debug mode

src/DebugSystem.cs

@@ -1,5 +1,7 @@
 // NOTE: these methods are very inefficient
 // this class should only be used in debugging contexts!!
+
+#if DEBUG
 namespace MoonTools.ECS
 {
 	public abstract class DebugSystem : System
@@ -24,3 +26,4 @@ namespace MoonTools.ECS
 		}
 	}
 }
+#endif