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/nested_class.wren

@@ -10,9 +10,9 @@ class Outer {
       }
     }
 
-    (new Inner).method
+    Inner.new().method
     IO.print(_field) // expect: outer
   }
 }
 
-(new Outer).method
+Outer.new().method