Add an autoformatter

[TheButlah]

I tried to create a PR and had a lot of trouble, because the code isn't formatted with rustfmt. I therefore needed to manually go through and delete everything that got formatted, so that I didn't submit a bunch of formatting noise and risk getting my PR rejected.

Would it be ok to add a rustfmt.toml file to the repo to indicate that the use of an autoformatter is permissible in PRs and define the format?

[bjoernQ]

With the default formatter settings, I get some changes in the macros, true. I'm wondering if you experienced more changes?

Anyway - sure - makes sense to add a rustfmt.toml - ideally this one https://github.com/esp-rs/esp-hal/blob/main/rustfmt.toml