StackOverFlowError when walking over set

The proposed fix for https://clojure.atlassian.net/browse/LOGIC-154#icft=LOGIC-154 fixes this.

core.logic no longer supports unification of sets so you're just seeing an error as result of a complete lack of support. I'm not against supporting basic unification of completely grounded sets, as then we only need to test for equality, but I'm inclined to make the system throw if any logic vars appear in the sets.