Function x509_signature::days_from_ymd

pub fn days_from_ymd(year: u16, month: u8, day: u8) -> Result<i32, Error>

We use our own version, instead of chrono, because:

• We can (and do) perform exhaustive testing of every possible input. The only possible inputs are (0, 0, 0) to (9999, 99, 99) inclusive, and we can (and do) test every single one of them in a reasonable amount of time.
• It avoids an unnecessary dependency, and thus prevents bloat.