Replace time with chrono

[mibmo]

Removes the old time dependency and switches all use to the more modern chrono crate. Tests pass but would love a proper review from someone familiar with the codebase. :)

I'm a bit concerned about the previous use of time::empty_tm(), as the documentation is a bit lacking, but I'm assuming that since it's mostly used for setting issuers' touched, it's used to represent the earliest possible time, so I've replaced it with chrono::DateTime::MIN_UTC.

Closes #9

[srhb]

Looks great, thank you! Your analysis about empty_tm seem correct to me (it's a more complicated version of "zero seconds from unix epoch", ie. zero days from epoch, zero weeks from epoch, zero ... from epoch) so your translation appears absolutely correct.