fold_dict: Use `IterableIterator` instead of `Iterator`.

The type `Iterator` does not allow us to loop over the iterator in a `for`
loop. but `IterableIterator` does.

web/src/fold_dict.ts

@@ -42,7 +42,7 @@ export class FoldDict<V> {
         return this._items.delete(this._munge(key));
     }
 
-    *keys(): Iterator<string> {
+    *keys(): IterableIterator<string> {
         for (const {k} of this._items.values()) {
             yield k;
         }