implement fold() on array::IntoIter to improve flatten().collect() perf

```
# old
test vec::bench_flat_map_collect                         ... bench:   2,244,024 ns/iter (+/- 18,903)

# new
test vec::bench_flat_map_collect                         ... bench:     172,863 ns/iter (+/- 2,141)
```

1 files changed, 6 insertions, 0 deletions

diff --git a/library/alloc/benches/vec.rs b/library/alloc/benches/vec.rs
index 91eec10d575..c93a493cadb 100644
--- a/library/alloc/benches/vec.rs
+++ b/library/alloc/benches/vec.rs
@@ -726,3 +726,9 @@ fn bench_dedup_old_100000(b: &mut Bencher) {
 fn bench_dedup_new_100000(b: &mut Bencher) {
     bench_vec_dedup_new(b, 100000);
 }
+
+#[bench]
+fn bench_flat_map_collect(b: &mut Bencher) {
+    let v = vec![777u32; 500000];
+    b.iter(|| v.iter().flat_map(|color| color.rotate_left(8).to_be_bytes()).collect::<Vec<_>>());
+}