allow usage of fold1

Author: hellow554

src/concat_impl.rs

@@ -18,5 +18,6 @@ pub fn concat<I>(iterable: I) -> I::Item
     where I: IntoIterator,
           I::Item: Extend<<<I as IntoIterator>::Item as IntoIterator>::Item> + IntoIterator + Default
 {
+    #[allow(deprecated)] //TODO: once msrv hits 1.51. replace `fold1` with `reduce`
     iterable.into_iter().fold1(|mut a, b| { a.extend(b); a }).unwrap_or_default()
 }

src/kmerge_impl.rs

@@ -213,6 +213,7 @@ impl<I, F> Iterator for KMergeBy<I, F>
     }
 
     fn size_hint(&self) -> (usize, Option<usize>) {
+        #[allow(deprecated)] //TODO: once msrv hits 1.51. replace `fold1` with `reduce`
         self.heap.iter()
                  .map(|i| i.size_hint())
                  .fold1(size_hint::add)