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.

[bibonix]

@yegor256 planning to take a look at this one and see whether the bench gap between the Stack and Vec can be narrowed by tightening up the iteration paths. Will start by getting the bench reproduced locally on master to confirm the numbers before touching anything. If anyone has context on what's already been tried or where the slowdown is suspected, please share before I dig in.

[bibonix]

Quick update before digging in: the baseline build is already red on master under current stable rustc (#[must_use] on trait-impl methods and an elided-lifetime warning are now hard errors, and #![deny(warnings)] turns them into compile failures). The last CI run on master failed for the same reason. I've filed #70 with the log excerpt and the build command. Standing down on this one until the baseline is green again, so the perf change can be verified against a working build.