added IntStack and started sketching out a mutable finate state machine version of the push parser
wip: creating CharacterStream, a mutable wrapper around Characters to be used by the new Matcher FSM
wip: CharacterStream basics in place, push/pop marks in the streams to follow
CharacterStream completed
wip porting constant matcher over to the new matcher (fsm) design
ported tests for ConstantMatcher to FSM push parser
added EOS to CharacterStream