Make Max and Min aggs use <=> instead of > or <

This comparator is more general, and accepts types like Arrays. This is really useful for vectors, so we can now take the max of vectors like [1, 2, 3] and [1, 2, 4].

Also added the same duplication case from Min for Max as well. I'm not certain why they were different, but I can't think of a reason why max should not return all maximum results, rather than an arbitrary one.

This change is motivated by our CS194-17 project and, uh, our solution won't run if this isn't patched. So we hope you find the change good!