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/super/super_in_top_level_function.wren

@@ -1,3 +1,3 @@
-new Fn {
+Fn.new {
   super.foo // expect error
 }