Refactor

Moved more responsibility onto Mapping
Add TODO.

@tooky do you see where this is going? We should be able to have a
Mapper that passes over the Test::Case/Step hierarcy and maps the
Test::Steps so they're ready for executing / skipping.

That will get rid of all references to the mappings from the runner.

Once we've done that, we can extend the Mapper to insert before / after
hooks.