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

bench/insert-bro-tree.sml

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