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/while/closure_in_body.wren

@@ -3,7 +3,7 @@ var list = []
 var i = 1
 while (i < 4) {
   var j = i + 1
-  list.add(new Fn { IO.print(j) })
+  list.add(Fn.new { IO.print(j) })
   i = i + 1
 }