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/closure/closed_closure_in_method.wren

@@ -1,3 +1,4 @@
+// TODO: Is this right? Shouldn't it resolve to this.local?
 var foo = null
 
 {
@@ -8,7 +9,7 @@ var foo = null
     }
   }
 
-  foo = new Foo
+  foo = Foo.new()
 }
 
 foo.method // expect: local