Add some utility methods for lists of sets.

src/advent/util.cr

@@ -0,0 +1,13 @@
+class Array(T)
+  def union
+    reduce(T.new) do |l, r|
+      l | r
+    end
+  end
+
+  def intersect
+    reduce do |l, r|
+      l & r
+    end
+  end
+end