Replace the Argument class from the tasks with the new lightweight objects.

Include a forward declaration of the lightweight object in the base header.

source/SqBase.hpp

@@ -141,6 +141,7 @@ namespace Sqrat {
     class Table;
     class Function;
     class StackStrF;
+    class LightObject;
 } // Namespace:: Sqrat
 
 /* ------------------------------------------------------------------------------------------------