Using process_skip in visits of ScopedAstVisitor

[ideadapt]

The following suggestion would simplify the logics implemented in existing visitors. It seems not to be required to have conditional logic in the leave methods of the visitors. Just retun process_skip to notify parent class.

Note: sub classes of ScopedAstVisitor therefore have only to return super.visit, process_skip/continue or similar is not required, since the ScopedAstVisitor is responsible for the scoping. He will decide when to leave a node or not.