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/call_other_method.wren

@@ -11,7 +11,7 @@ class Derived is Base {
   }
 }
 
-(new Derived).bar
+Derived.new().bar
 // expect: Derived.bar
 // expect: Base.foo