Half-hearted update to 2018 edition

[Follpvosten]

Disclaimer: This was done as I was bored waiting at the airport, so it might not be perfect/complete yet.

Since proc_macro was stabilized recently, I thought it would be a nice idea to make this work on stable. Well, surprise, proc_macro_diagnostics is still an unstable feature. But hey, it works on the current nightly, so that's something, I guess?

Apart from the obvious, I also applied rustfmt and fixed the one thing Clippy complained about.

[zmitchell]

I've been meaning to update this (and the article) to Rust 2018 at some point in the near future. I'll take a closer look at this when I update the article. Thanks for your contribution!