Convert all fns to block arg syntax.

test/super/closure.wren

@@ -4,9 +4,7 @@ class Base {
 
 class Derived is Base {
   getClosure {
-    return fn {
-      return super.toString
-    }
+    return new Fn { super.toString }
   }
 
   toString { return "Derived" }

test/super/super_in_top_level_function.wren

@@ -1,3 +1,3 @@
-fn {
+new Fn {
   super.foo // expect error
 }