Adding Constant class and ProductionReference as a superclass of TermCons

This allows for better error reporting in the new parser when having ambiguities.
It will also allow for more simple disambiguation filters.

@dwightguth please review. We talked about this change a while back, and now I got to it.
Basically now the new parser outputs only two types of objects: TermCons and Constant, bot having a reference to the production from which they come from.
I'm not comfortable with this constructor. It should be an absolute that all ProductionReferences have a non-null production