add function to quad tree to turn items into strings (for later saving to file)

fcore/quad-tree.sml

@@ -500,4 +500,39 @@ struct
     in
       Vector.concat vec
     end
+
+  fun toStringFolder ({x, ex, y, ey, data = {r, g, b, a}}, acc) =
+    let
+      val item = String.concat
+        [ "{"
+        , Int.toString x
+        , " "
+        , Int.toString y
+        , " "
+        , Int.toString ex
+        , " "
+        , Int.toString ey
+        , " "
+        , Int.toString r
+        , " "
+        , Int.toString g
+        , " "
+        , Int.toString b
+        , " "
+        , Int.toString a
+        , " }"
+        ]
+    in
+      item :: acc
+    end
+
+  fun toString (squares, size) =
+    let
+      val qtree = buildTree (0, 0, size, squares)
+      val bintree = merge (qtree, squares)
+
+      val vec = BinTree.foldr (toStringFolder, bintree, [])
+    in
+      Vector.concat vec
+    end
 end