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/static_field/nested_class.wren

@@ -25,10 +25,10 @@ class Outer {
       }
     }
 
-    (new Inner).instanceMethod
+    Inner.new().instanceMethod
     IO.print(__field) // expect: outer
   }
 }
 
 Outer.staticMethod
-(new Outer).instanceMethod
+Outer.new().instanceMethod