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/reuse_closure_slot.wren

@@ -3,7 +3,7 @@
 
   {
     var a = "a"
-    f = new Fn { IO.print(a) }
+    f = Fn.new { IO.print(a) }
   }
 
   {