Get binary_trees benchmark working.

Wren is actually doing well in it:

wren      mean: 1.9441 median: 1.9428 std_dev: 0.0260
lua       mean: 3.5992 median: 3.6033 std_dev: 0.0156
python    mean: 3.6667 median: 3.7097 std_dev: 0.1340
ruby      mean: 1.3941 median: 1.3914 std_dev: 0.0091

benchmark/binary_trees.wren

@@ -0,0 +1,60 @@
+// Ported from the Python version.
+
+class Tree {
+  this new(item, depth) {
+    _item = item
+    if (depth > 0) {
+      var item2 = item + item
+      depth = depth - 1
+      _left = Tree.new(item2 - 1, depth)
+      _right = Tree.new(item2, depth)
+    }
+  }
+
+  check {
+    if (_left == null) {
+      _item
+    } else {
+      _item + _left.check - _right.check
+    }
+  }
+}
+
+var minDepth = 4
+var maxDepth = 14
+var stretchDepth = maxDepth + 1
+
+var start = OS.clock
+
+io.write("stretch tree of depth " + stretchDepth.toString + "\t check: " +
+    Tree.new(0, stretchDepth).check.toString)
+
+var longLivedTree = Tree.new(0, maxDepth)
+
+// iterations = 2 ** maxDepth
+var iterations = 1
+var d = 0
+while (d < maxDepth) {
+  iterations = iterations * 2
+  d = d + 1
+}
+
+var depth = minDepth
+while (depth < stretchDepth) {
+  var check = 0
+  var i = 1
+  while (i < iterations + 1) {
+    check = check + Tree.new(i, depth).check + Tree.new(-i, depth).check
+    i = i + 1
+  }
+
+  io.write((iterations * 2).toString + "\t trees of depth " + depth.toString +
+      "\t check: " + check.toString)
+  iterations = iterations / 4
+  depth = depth + 2
+}
+
+io.write("long lived tree of depth " + maxDepth.toString + "\t check: " +
+    longLivedTree.check.toString)
+
+io.write("elapsed: " + (OS.clock - start).toString)