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/field/in_fn_in_static_method.wren

@@ -1,5 +1,5 @@
 class Foo {
   static bar {
-    new Fn { _field = "wat" } // expect error
+    Fn.new { _field = "wat" } // expect error
   }
 }