Parsing expressions keep track of the previous parsing expression during
recursive descent. This enables StrMatch to mark terminal for
suppression if called from Sequence.

Fixed bug in parser model construction from python grammar description
with direct rule call.

Converting terminal to a string as a default action.

SemanticAction now have a default behaviour which will reduce overhead
in some common cases.

Arpeggio now supports both python 2 and 3.

This works for rules that are referenced from the rule which created
NonTerminal.