* lib/adlint/cc1/expr.rb : Add event notifications in order to cover all
  kinds of expression evaluations.
* lib/adlint/cc1/interp.rb : Ditto.
* lib/adlint/cc1/mediator.rb : Ditto.

* lib/adlint/cc1/value.rb : Add ScalarValueFactory#of_null method to
  confine a knowledge about representation of NULL to one place.