Design ICU4X ZoneInfo data structs for TZDB data

[nordzilla]

The TzdbDataProvider from #998 will transform TZDB data into ICU4X data.

This data will require the following information for each time zone:

1. IANA TimeZone Identifier
2. DST variant, transition times, and GMT offset for each relevant year.

[sffc]

Actually I don't think this depends on the tzdb-provider crate. The struct definitions should live in the crate that uses them, i.e., the time zone arithmetic crate.

[nordzilla]

Good point. I'll remove the dependency. Thank you.