Add append to treap structure

treap.hpp

@@ -110,6 +110,9 @@ public:
 		two.first = join(two.first, l);
 		root = join(two);
 	}
+	void append(string s) {
+		insert(size(), s);
+	}
 	// Line removal
 	void remove(count_type k) {
 		auto two = split(root, k+1);