v1.3 - better ZoneId support using ZoneManager interface

• 1.3 (2020-11-30, TZ DB version 2020d)
  • Minor tweaks to silence clang++ warnings.
  • Create new ZoneManager interface (pure virtual) which is now the
    non-templatized parent to both BasicZoneManager and
    ExtendedZoneManager. Allows ZoneManager to be passed around
    poloymorphically as a pointer or reference.
  • Fix broken ZoneManager::indexForZoneName() and
    ZoneManager::indexForZoneId() caused by incorrect implementations in
    BasicZoneRegistrar and ExtendedZoneRegistrar.
  • Generate compile-time zoneIds for all zones in the form of
    zonedb::kZoneId{Zone_Name} and zonedbx::kZoneId{Zone_Name} (e.g.
    zonedb::kZoneIdAmerica_Los_Angeles). Can be given directly to
    ZoneManager::createForZoneId().
  • Add constructors to TimeZoneData to allow initializers to set
    union members. Useful for initializing arrays of TimeZoneData.
  • Add ManualZoneManager implementation of ZoneManager which implements
    only createForTimeZoneData(). Useful in applications which support only
    TimeZone::kTypeManual (fixed std and dst offsets) due to memory
    constaints.
  • Add documentation of TimeZoneData, TimeZone::toTimeZoneData(), and
    ZoneManager::createFromTimeZoneData() to USER_GUIDE.md. Looks like I
    added the class in v0.5 but forgot to document it.
  • Implement LocalDateTime::compareTo() using only its components instead
    of internally converting to epochSeconds. Not all LocalDateTime can be
    represented by an epochSeconds, so this change makes the algorithm more
    robust. The semantics of the method should remain unchanged.
  • Update the doxygen docs of the compareTo() methods of LocalDateTime,
    LocalTime, LocalDate, OffsetDateTime and ZonedDateTime to clarify
    the semantics of those operations.