add benchmarks for tiny_rope in bench folder

bench/rope_svelte.sml

@@ -0,0 +1,13 @@
+structure RopeSvelte: TRANSACTION =
+struct
+  type t = TinyRope.t
+  val empty = TinyRope.empty
+  val insert = TinyRope.insert
+  val delete = TinyRope.delete
+  val toString = TinyRope.toString
+  val txns = SvelteComponent.txns
+end
+
+structure Main = Run(RopeSvelte)
+
+val _ = Main.run ()