The8472 e015e9da71 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)
```
2021-07-24 19:24:11 +02:00
..