Run rustfmt and require it in CI

[madsmtm]

The changes I've made can be seen here - the rest is done automatically by rustfmt

[SSheldon]

I'll be honest, I think rustfmt does some ugly things, so I'm not in any rush to do this. If there's a lot of changes being merged from multiple contributors, I think this makes sense. But as you can see that's not the case here because I am neglectful. I will keep this in mind to merge if development ramps back up here.

[madsmtm]

I agree that it's formatting is suboptimal, but it's better than nothing, and some things it definitely does better (like putting the explicit "C" on extern items, considering the maybe upcoming "C-unwind").

Which parts is it specifically that you have a gripe with? rustfmt's behaviour can easily be changed to suit your preferred style, see this.

But yeah, no rush on this one.