Store datetimes with fixed offset

[sharkdp]

This changes a couple of things regarding datetime handling:

• We don't store utc datetime and offset separately, but as a single DateTime. This not just fixes a bug, but also feels much cleaner overall.
• Do not ignore offsets in format_datetime, fixes Support for RFC 3339 in target time zone #376
• Fix crashes for out-of-range values in from_unixtime
• datetime(…) will now return datetimes with the specified UTC offset (not the local UTC offset)
• from_unixtime(…) will now return datetimes in the local timezone, not in UTC

[sharkdp]

@eminence Do you remember why you chose to store datetime in UTC + offset separately?

[eminence]

No, not really. I think the initial version didn't support any datetime conversions, so storing a DateTime<Utc> was a natural fit, and it didn't evolve in the best way when Tz conversions were added. I may have also been operating under [a potentially faulty] assumption that it would be easier to reason about datetime operations is everything was done in Utc (and then only converted to something else for printing).

However skimming your new version here, it does look less complicated 👍

[sharkdp]

Thank you!