Stack::into_iter() is slower than Vec::into_iter()

[yegor256]

According to the results of vs_vec.rs bench, we are slower in iterating than Vec:

$ rustup run nightly cargo bench
...
     Running benches/vs_vec.rs (target/release/deps/vs_vec-903d5e84934e313a)

running 2 tests
test stack_push_and_pop ... bench:       6,058 ns/iter (+/- 449)
test vec_push_and_pop   ... bench:       3,425 ns/iter (+/- 138)

The problem is in the into_iter() method. Let's figure out where is the lag and fix.