Implement FlattenOk::fold

Author: kinto-b

src/flatten_ok.rs

@@ -72,6 +72,17 @@ where
         }
     }
 
+    fn fold<B, F>(self, init: B, mut f: F) -> B
+    where
+        Self: Sized,
+        F: FnMut(B, Self::Item) -> B,
+    {
+        self.iter.fold(init, |acc, x| match x {
+            Ok(it) => it.into_iter().fold(acc, |a, o| f(a, Ok(o))),
+            Err(e) => f(acc, Err(e)),
+        })
+    }
+
     fn size_hint(&self) -> (usize, Option<usize>) {
         let inner_hint = |inner: &Option<T::IntoIter>| {
             inner