* Create interface to allow keyparser description to be built up

* Create interface to allow keyparser description to be built up
  incrementally.  This aids in development and debugging of
  descriptions.

* Modify indentation computation so that comment lines are indented
  properly.  Previously they were indented just like code lines.

* Change definition of MATCH-HEADER so that it accepts the keyparser
  stack as an argument.  Also open up the stack-entry abstraction.
  These changes allow MATCH-HEADER to use contextual information in
  its decision-making process.

* Fix some problems in the VHDL implementation.  Use new MATCH-HEADER
  extension to distinguish two cases of FOR keyword that are otherwise
  syntactically identical.