Since 1.37.0, Rust 2015 also supports Kleene ? repetition operator

[Centril]

As of rust-lang/rust#60932.

Changes need to be done in:

• https://doc.rust-lang.org/nightly/edition-guide/rust-2018/macros/at-most-once.html
• https://doc.rust-lang.org/nightly/edition-guide/rust-2018/edition-changes.html