Make constructors just methods.

* Eliminate "new" reserved word.
* Allow "this" before a method definition to define a constructor.
* Only create a default constructor for classes that don't define one.

test/benchmark/binary_trees.wren

@@ -1,13 +1,13 @@
 // Ported from the Python version.
 
 class Tree {
-  new(item, depth) {
+  this new(item, depth) {
     _item = item
     if (depth > 0) {
       var item2 = item + item
       depth = depth - 1
-      _left = new Tree(item2 - 1, depth)
-      _right = new Tree(item2, depth)
+      _left = Tree.new(item2 - 1, depth)
+      _right = Tree.new(item2, depth)
     }
   }
 
@@ -27,9 +27,9 @@ var stretchDepth = maxDepth + 1
 var start = IO.clock
 
 IO.print("stretch tree of depth ", stretchDepth, " check: ",
-    new Tree(0, stretchDepth).check)
+    Tree.new(0, stretchDepth).check)
 
-var longLivedTree = new Tree(0, maxDepth)
+var longLivedTree = Tree.new(0, maxDepth)
 
 // iterations = 2 ** maxDepth
 var iterations = 1
@@ -41,7 +41,7 @@ var depth = minDepth
 while (depth < stretchDepth) {
   var check = 0
   for (i in 1..iterations) {
-    check = check + new Tree(i, depth).check + new Tree(-i, depth).check
+    check = check + Tree.new(i, depth).check + Tree.new(-i, depth).check
   }
 
   IO.print((iterations * 2), " trees of depth ", depth, " check: ", check)