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.
When we switch to Java8, we should have a look at something like: https://blogs.oracle.com/java-platform-group/entry/java_8_s_new_type
Might save us a lot of trouble if we annotate things that we can expect them to be null or not.