add cargo fmt to CI, run initial format

[syphar]

Coming from the comment by @Enselic in #526: #526 (comment)

I agree with should rustfmt all code and enforce it in CI though. More or less all other projects in the Rust ecosystem does that, and the upsides far outweighs the downside IMHO.

I also added the initial format commit to the ignore-blame file: https://docs.github.com/en/repositories/working-with-files/using-files/viewing-a-file#ignore-commits-in-the-blame-view

[Enselic]

Looks great, thanks! Bonus points for .git-blame-ignore-revs.

I've brought this up every now and then and have heard no strong objections from other maintainers.

Let's merge!