Jiff spike

[ctm]

Try to quickly replace chrono and chrono-tz with jiff.

Most likely, if I can do this in under a day, it will be worthwhile. Furthermore, if I'm going to do it, I should do it sooner rather than later due to the serialization format difference. OTOH, I've spent well under an hour looking at jiff and this isn't important enough to merit high priority.

Thanks, Ed!