Visitor trait `visit_arg` is never called

[jvllmr]

When using the Visitor trait I realized that the default values of arguments are never visited. This is because the visit_arg function is never actuall called: https://github.com/RustPython/Parser/blob/4588ea5c3e6327009640e7c9c89eb6fa9220358e/ast/src/gen/visitor.rs#L737

I also could not see an implementation for visiting the default value of an arg.