Equality by key?

[tantaman]

• Along the lines of Less Comparators? #12

We could do equality by key rather than requiring comparators.

I think equality by key makes sense internally -- we just create the key based on the index of the item in the source -- and this is never exposed to users.

Equality by key for the user seems overall less flexible than a comparator. An upside is equality by key doesn't require lookup via a fully constructed, or partially constructed, object. A downside is that compound orderings would require encoding their fields into the key via string concatenation in a way that is lexiographically sorted as they expect.

Maybe comparators should be Optional<T> rather than <T>? Or a generic C extends Optiona<T>? As not all fields are often required for the comparator. This gives us the best of both worlds (comparator vs key) with some added type complexity.