Add VisitorAdaptorgen.g for Ceylon interoperability

This adds a VisitorAdaptorgen grammar that will in turn generate a VisitorAdaptor class which extends Visitor, but offers distinct method names for each Node type
(since Visitor's `visit(AnyOfAThousandSubtypes that)` can't be used from Ceylon).

CC [ceylon-ide-eclipse#385](https://github.com/ceylon/ceylon-ide-eclipse/issues/385)