Copy the new Vec::retain implementation into the retain_mut function

[Kerollmops]

In some previous PRs (#83, #85), we introduced the retain_mut helper function that permits the modification of the visited elements by giving a &mut of the elements. Now that the standard library Vec::retain function was rewritten and gives more than 20% performance improvement, it would be cool to patch our own function too!

[upsuper]

FWIW, you can use my crate retain_mut. It hasn't updated the code, but I do plan to when the change reaches stable.

(Actually the change to Rust std was motivated by my inspect of the retain_mut crate for which I raised this inefficiency in a Rust group and another member decided to fix it :))