Support inserting a treap into another

treap.hpp

@@ -110,6 +110,12 @@ public:
 		two.first = join(two.first, l);
 		root = join(two);
 	}
+	void insert(int k, treap t) {
+		// TODO test
+		auto two = split(root, k);
+		two.first = join(two.first, t.root);
+		root = join(two.first, two.second);
+	}
 	void append(string s) {
 		insert(size(), s);
 	}