Time zones

[jwise]

I think this is a good chunk of work, but no individual step is all that complicated. Basically, the steps involved are:

• [x] Come up with a on-disk format for tzinfo storage. Probably it will be as part of the resource bundle, rather than in microflash.
• [x] Bring in https://github.com/eggert/tz as a submodule.
• [x] Add a compiler from zic-compiled tzinfo to a packed format that's a little more efficient than tzinfo raw files.
• [x] Modify the localtime.c in eggert/tz to support loading a timezone from a system resource. Ideally, modify it to not have to load the whole timezone into RAM at once (expensive: costs 6 kB for some time zones!), and to iteratively read() to get what it needs.
• [x] Write UI to change the timezone.
• [x] Write logic to take timezone from phone.
• [x] Write backend to store time preferences (while we're at it, implement 24hr preference) in a rdb prefdb.
• [ ] Scrub APIs for things that we do wrong right now - clock_get_timezone, etc.
• [ ] Fire tick events for when the timezone changes. Or just don't put the time in the status bar in the settings menu. 🙈

[jwise]

I'm going to defer the 'scrub API' work for later, because I'm feeling burned out on this. But I think this is done.