add insert benchmark comparing StringSet to BroTree (StringSet is just a few ms faster for this)

bench/insert-string-set.sml

@@ -0,0 +1,11 @@
+structure InsertStringSet =
+struct
+  fun main () =
+  let
+    val endTrie = Vector.foldl StringSet.insert StringSet.empty WordsList.words
+  in
+    ()
+  end
+end
+
+val _ = InsertStringSet.main ()