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.
Fixing Copyright comment.
Fixing FlattenSyntax for the new Constant class.
Amending some of Dwight's comments.
Fixing the final example in the unit tests for the new parser.
Minimize the final diff changes.
Removing one of the Constant constructor.
Merge pull request #894 from kframework/addConstant

Add Constant class.