MethodCase is responsible for logging known case failures, instead of relying on the caller to do so.
Cases and Fixtures use a common Result type containing only counts.  Exceptions are only reported through the Listener.
Refactor Runner to reveal symmetry between running all Fixtures in a Convention vs all Cases in a Fixture.
Move Runner's execution of fixtures into Fixture.  Move Runner's execution of Conventions into Convention.  Remove Runner now that it is empty.