This implements the loading of the tzdata.zi file and store its contents
in the tzdb struct.
This adds all required members except:
- the leap seconds,
- the locate_zone, and
- current_zone.
The class time_zone is incomplete and only contains the parts needed for
storing the parsed data.
The class time_zone_link is fully implemented including its non-member
functions.
Implements parts of:
- P0355 Extending <chrono> to Calendars and Time Zones
- P1614 The Mothership has Landed
Implements:
- P1982 Rename link to time_zone_link
