leap seconds

[est31]

I was wondering about adopting the time crate for two libraries that I maintain, rcgen and yasna. However, I've seen that there is no handling of leap seconds in time. I'm still investigating how important they are for rcgen and yasna use cases, but generally single seconds can mean a lot. Even if yasna and rcgen benefit only little from leap seconds, I think it's important to have them in a general purpose time handling library. Not everyone uses google's smeared seconds standard :).

[jhpratt]

Leap seconds inherently rely on the time zone database, which is tracked in #193. Properly handling them is extremely difficult; chrono doesn't even handle them properly.

As an aside, the issue with LetsEncrypt has nothing whatsoever to do with leap seconds.