into_group_map: make code more idiomatic

Author: Kek5chen

src/group_map.rs

@@ -13,13 +13,10 @@ where
     I: Iterator<Item = (K, V)>,
     K: Hash + Eq,
 {
-    let mut lookup = HashMap::new();
-
-    iter.for_each(|(key, val)| {
-        lookup.entry(key).or_insert_with(Vec::new).push(val);
-    });
-
-    lookup
+    iter.fold(HashMap::new(), |mut acc, (key, val)| {
+        acc.entry(key).or_default().push(val);
+        acc
+    })
 }
 
 pub fn into_group_map_by<I, K, V, F>(iter: I, mut f: F) -> HashMap<K, Vec<V>>