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/language/for/only_evaluate_sequence_once.wren

@@ -1,4 +1,4 @@
-var f = new Fn {
+var f = Fn.new {
   IO.print("evaluate sequence")
   return [1, 2, 3]
 }