Currently icu_time::DateTime does not implement PartialOrd nor Ord.

Implementing the traits for it should be very simple, since Date<A> and Time already implement them - we'd only need to order by the date first and then the time.

Furthermore, ZonedDateTime also implements PartialOrd + Ord, so I don't see why DateTime couldn't.

If there are no objections to this, I'd love to make a PR for it :)