I don't think we should expose tokens as an API
yet. They're an implementation detail, not used
much, and the AST should be able to reflect all
the information that can be found in the tokens.
The methods of AbstractNode can be pulled down
to AbstractJjtreeNode and made protected.
* Move AbstractModelicaRule into the `rule` package
* Make the visitor interface mention only published types (ie not
AbstractModelicaNode)
* Other cleanups(eg make interfaces extend ModelicaNode)
