implement 'exists' function on bro-tree as well, and benchmark it compared to string-set

bench/build-exists-string-set.sml

@@ -0,0 +1,33 @@
+structure BuildExistsStringSet =
+struct
+  fun helpExists (pos, trie, acc) =
+    if pos = Vector.length WordsList.words then
+      acc
+    else
+      let
+        val word = Vector.sub (WordsList.words, pos)
+        val newAcc = StringSet.exists (word, trie)
+        val acc = newAcc orelse acc
+      in
+        helpExists (pos + 1, trie, acc)
+      end
+
+  fun exists trie = helpExists (0, trie, true)
+
+  fun main () =
+    let
+      val endTrie =
+        Vector.foldl StringSet.insert StringSet.empty WordsList.words
+
+      val startTime = Time.now ()
+      val wordsExist = exists endTrie
+      val finishTime = Time.now ()
+
+      val searchDuration = Time.- (finishTime, startTime)
+      val searchDuration = Time.toString searchDuration ^ "\n"
+    in
+      print searchDuration
+    end
+end
+
+val _ = BuildExistsStringSet.main ()