[stdlib] Fix iterator ergonomics

stdlib/src/builtin/builtin_list.mojo

@@ -147,6 +147,10 @@ struct _VariadicListIter[type: AnyTrivialRegType]:
     fn __len__(self) -> Int:
         return len(self.src) - self.index
 
+    @always_inline
+    fn __bool__(self) -> Bool:
+        return self.__hasmore__()
+
 
 @register_passable("trivial")
 struct VariadicList[type: AnyTrivialRegType](Sized):
@@ -258,6 +262,10 @@ struct _VariadicListMemIter[
     fn __len__(self) -> Int:
         return len(self.src[]) - self.index
 
+    @always_inline
+    fn __bool__(self) -> Bool:
+        return self.__hasmore__()
+
 
 # Helper to compute the union of two origins:
 # TODO: parametric aliases would be nice.