Timestamp::saturating_add misbehaves if the span contains calendar units

[FeldrinH]

let now = Timestamp::now();
let span = 1.day();
println!("{}", now.saturating_add(span));

Expectation: Error, panic or some other indication that addition failed.

Actual result: 9999-12-30T22:00:00.999999999Z

[BurntSushi]

Thank you for catching this. This was a bad oversight on my part. I think I had written the saturating method routine before I smoothed out the error cases for checked arithmetic. This will indeed mean that Timestamp::saturating_add should return an error. I might make it panic in the interim before a jiff 0.2 release.

This also motivates #21, because with an "absolute" duration, we can provide infallible saturating arithmetic on Timestamp.