tzdb support

[jhpratt]

This is a general tracking issue for all support related to the IANA Time Zone database.

At some point in the future, my hope is to have full support for the tzdb via a ZonedDateTime struct and a Period type.

General inspiration could come from C++20's proposed API for timezones [ref]. It should be possible to build a static/const singleton by using a build script.

[jhpratt]

The tzfile crate was recently brought to my attention and may be a decent starting point once I get around to implementing this.

[Yuri6037]

I've just begun implementing support for tz database without relying on chrono dependency.

EDIT: link to repository for anyone interested: https://github.com/Yuri6037/time-tz