v2.1.0 - unified links; simplify TimeZone using ZonedExtra class

• 2.1.0 (2023-01-29, TZDB version 2022g)
  • There are a handful API breaking changes in this release in the pursuit of
    simpler and cleaner code. See the following for more info:
    • Migrating to v2.1
    • ZonedExtra in the User Guide
    • Unified Links in the User Guide
  • Potentially Breaking: zonedb,zonedbx
    • Rename kPolicyXxx to kZonePolicyXxx for consistency. These are
      expected to be used only internally, so shouldn't cause external
      breakage.
  • Breaking: TimeZone.h
    • Replace 3 separate extraction methods in TimeZone with a new
      ZonedExtra class
    • Removed: TimeZone::getUtcOffset()
      • Replaced by: ZonedExtra::timeOffset()
    • Removed: TimeZone::getDeltaOffset()
      • Replaced by: ZonedExtra::dstOffset()
    • Removed TimeZone::getAbbrev()
      • Replaced by: ZonedExtra::abbrev()
      • ZonedExtra::abbrev() returns pointer to a local string buffer
        instead of a transient buffer deep inside Transition object.
      • TimeZone becomes closer to being thread-safe
  • New Class: ZonedExtra.h
    • ZonedExtra::forEpochSeconds(epochSeconds, tz)
      • Create instance from epochSeconds and time zone.
    • ZonedExtra::forLocalDateTime(ldt, tz)
      • Create instance from LocalDateTime and time zone.
  • Potentially Breaking: Unified Links
    • Links are now first-class citizens, exactly the same as Zones.
    • Unify "fat links" and "symbolic links" into a single implementation.
    • Remove "thin links" to simplify the code.
    • TimeZone class simplified
      • Removed followLink flag on various methods.
      • Only 2 methods apply to Links: isLink() and
        printTargetNameTo().
  • Simplify ZoneProcessors
    • ZoneProcessor.h, ExtendedZoneProcessor.h, BasicZoneProcessor.h
    • Remove: getUtcOffset(), getDeltaOffset(), getAbbrev()
    • Replaced by: findByLocalDateTime(), findByEpochSeconds()
    • These are internal helper methods not intended for public consumption.
  • Unit tests
    • Migrate most unit tests to use the smaller, testing zone databases at
      testing/tzonedb/ and testing/tzonedbx/.
      • Reduces maintenance cost of various hand-crafted ZoneInfo and
        ZonePolicy entries for unit tests.
      • Can test against real timezones with predictable behavior.