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/closure/shadow_closure_with_local.wren

@@ -1,6 +1,6 @@
 {
   var foo = "closure"
-  new Fn {
+  Fn.new {
     {
       IO.print(foo) // expect: closure
       var foo = "shadow"