merge 2.3.0 into master

[bxparks]

Major Changes in v2.3: Add CompleteZoneProcessor, CompleteZoneManager,
and the zonedbc database to support all timezones, for all transitions defined
in the IANA TZ database ([1844,2087]), and extending the validity of timezone
calculations from [2000,10000) to [0001,10000).

Migration: MIGRATING.md

Changelog

• 2.3.0 (2023-06-27, TZDB version 2023c)
  • Create CustomZoneRegistry example
    to illustrate how to use a custom registry.
  • Update BasicZoneManager to detect gaps.
    • Allows AceTimeValidation/validation tests to pass.
    • Still not able to distinguish between exact and overlap though.
  • Allow multi-character ZoneRule.letter in BasicZoneProcessor.
    • Regenerate zonedb which enables 2 more zones: /Africa/Windhoek,
      and America/Belize
    • Increases zonedb by 150-200 bytes.
  • Rename BrokerFactory to ZoneInfoStore for better self-documentation.
    • This is an internal implementation detail. Downstream clients should
      not be affected.
  • ZoneContext
    • Move ZoneContext, letters[], fragments[] into PROGMEM.
    • Move zonedbXxx::kTzDatabaseVersion string into PROGMEM, and change
      type to const __FlashString*.
    • Create ZoneContextBroker around ZoneContext, for consistency
      with all other zoneinfo data structures. Merge ZoneContext.h into
      ZoneInfo.h.
    • Add startYearAccurate and untilYearAccurate to ZoneContext which
      define the interval of accurate transitions.
    • Saves around 150-200 bytes of RAM on AVR processors,
      200-350 bytes of RAM on ESP8266.
  • Cleanly separate zoneinfo storage classes from their brokers classes.
    • Three storage implementations instead of one: zoneinfolow,
      zoneinfomid, zoneinfohigh, supporting low, middle, and high time
      resolutions.
    • zoneinfohigh was created to support CompleteZoneProcessor,
      CompleteZoneManager and the zonedbc database.
    • zoneinfomid initially used in ExtendedZoneProcessor, but now
      unused.
    • Merge zoneinfo/ZonePolicy.h into zoneinfo/ZoneInfo.h.
    • Convert structs in zoneinfo/ZoneInfo.inc into templates, with
      synthetic typename S selector. Merge ZoneInfo.inc into
      ZoneInfo.h.
  • Support timezones before 1972 using CompleteZoneManager,
    CompleteZoneProcessor, and zonedbc database
    • Before standardizing on UTC around 1970, many timezones had local
      times offsets from UTC which require one-second resolution instead of
      one-minute resolution. This requires changing many internal variables
      from int16_t to int32_t.
    • Change TimeOffset to support one-second resolution using int32_t.
    • Rename BasicZone::stdOffseMinutes() to BasicZone::stdOffset()
      which returns a TimeOffset object.
    • Rename ExtendedZone::stdOffseMinutes() to
      ExtendedZone::stdOffset() which returns a TimeOffset object.
    • Leave BasicZoneProcessor using one-minute resolution because its
      algorithm has an inherent limitations which cannot handle many
      timezones before 1972, so no need to convert int16_t to int32_t
      fields.
    • Add CompleteZoneManager, CompleteZoneProcessor,
      CompleteZoneProcessorCache, CompleteZone
    • Add zonedbc database which is valid from [1800,10000), which
      includes all transitions in the TZDB since the first transition is
      1844.
    • Add scope=complete to access the zonedbc database.
  • Zone Processor with graceful degradation
    • Remove range checks against ZoneContext.startYear() and
      ZoneContext.untilYear()
    • Replace with LocalDate::kMinYear and LocalDate::kMaxYear, mostly
      for formatting reasons (prevent negative
      years, and years with more than 4 digits).
    • The zone processors will always return something reasonble across the
      entire int16_t range.
    • Only the accuracy suffers outside of the startYearAccurate() and
      untilYearAccurate() limits.
    • Along with v2.2 which always generates anchor rules for all zone
      policies, the startYearAccureate() and untilYearAccurate() allows
      for graceful degradation of classes like ZonedDateTime for years
      outside of this accuracy range.
  • Rename src/tzonedb* directories
    • to src/zonedb*testing for consistency with other acetime libraries