Enforce formatting in CI

[hannobraun]

#147 formats the code using cargo fmt, but doesn't update the CI build to enforce that. I think this should be done, as otherwise the formatting is going to drift again, over time.

When doing that, it might be worth deciding whether to include any formatting configuration that deviates from the default. Personally, I'm always changing the maximum line width to 80 characters (max_width = 80), as any other value is objectively too long or too short (no doubt about it 😁).

Blocked on #128.