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/nonlocal/undefined.wren

@@ -1,7 +1,6 @@
-var fn = new Fn {
+var fn = Fn.new {
   IO.print(Foo)
   IO.print(Bar)
 }
 
-// expect error line 7
-// expect error line 7
+// expect error line 6