Remove Comparator#score_datasets implementation

- the default implementation should be handled in the runner for the
  sake of speed
- added Comparator#type
- require subclasses of Comparator to define both score_datasets and
  score_dataset if they don't define the score method
Add :equal_to operator to Compare

- using :equal_to in Compare switches to advanced comparator mode
- use to_expr in Compare#score instead of name
- implement sorted comparisons in Compare#score_datasets and
  Compare#score_dataset for :equal_to