Continue validating the input symbol limiting parsers
o Rename ..._discard to ..._drop
o Add typecode to base state signature, to improve usability of class
o Correct evaluation of limit predicate; path includes machine context,
  but the transition 'decide' predicate path does not (evaluated in
  parent matchine's context)
o Fix move_if decide implementation to avoid variable name reuse, and
  to allow both source and destination specification.
o Implement and test Logix CIP Extended Path Segments parser