Refactor: Extract function util::c_cmp_to_ordering

[emlun]

I noticed some repeated code, and also added a test for good measure.

[ehuss]

Thanks!

[emlun]

You're welcome!

By the way, I also experimented a bit with adding a trait OrdC { fn c_cmp(...) -> c_int } and an impl<T: OrdC> Ord for T, but it seems like that's not allowed by Rust's orphan rules (it would work if Ord had been defined in this crate). If you have any ideas on how something like that could be doable, I'd be happy to experiment a bit more with it to see what happens.