add benchmark files for line_gap.sml and adjust the Makefile in /bench/ directory to use them

bench/line_gap_rust.sml

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