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/implicit_name.wren

@@ -11,6 +11,6 @@ class Derived is Base {
   }
 }
 
-(new Derived).foo
+Derived.new().foo
 // expect: Derived.foo
 // expect: Base.foo