[fine] Some resilience, a design improvement for iterator

fine/tests/expression/alternates.fine

@@ -83,16 +83,20 @@ fun more_examples(weapon: MeleeWeapon or RangedWeapon) -> f64 or () {
 }
 
 // Some fun with iterators
+class Finished {}
+let FINISHED = new Finished {}
 
 class Iterator {
   current: f64;
 
-  fun next(self) -> f64 or () {
+  fun next(self) -> f64 or Finished {
     if self.current < 10 {
       let result = self.current;
       self.current = self.current + 1;
       return result;
     }
+
+    FINISHED
   }
 }