subsetmaker: rework the antichain generation to be usable Before this, antichain computation can run for 10s of hours without completion in sight. We use a more direct approach in the computation to keep the computation in complexity in check. With good result. We can now have a full antichain computation on mozilla-try in about one minute. Which is usable. Differential Revision: https://phab.mercurial-scm.org/D12396