Stub out tzdb support for Windows and POSIX tz string

[nekevss]

Builds on #99.

This PR adds resolving the POSIX tz string and should allow handling time zones on Windows via jiff_tzdb as a source, which should make the stub overall somewhat stable overall until a different solution can be put into place.

[jasonwilliams]

I think it might be good to have some description on what the Tzif struct is as there's currently no documentation written above it. I assume it's short for TimeZoneInFo, not sure though.

I think the same could be said for some of the other new structs in tzdb.rs too like LocalTimeRecord and TransitionTimeSearch etc..

I'm not sure im understanding the slim format difference, the resulting Tzif result looks to be the same size (552 bytes) whichever format is used.

Tests look good

[nekevss]

Very true. I'll update the docs now.

[nekevss]

I'm not sure im understanding the slim format difference, the resulting Tzif result looks to be the same size (552 bytes) whichever format is used.

The "slim" vs. "fat" format is laid out by zic. Basically, all OSs are compiled using the "fat" format, but if someone does a custom compilation, then a smaller range can be set for which the transition times are computed.

The zic docs can be found here. Primarily, zic's bloat and range flags.