1.7.1 - add SAMD21, SAMD51 boards to Tier 1; clean up Flash.h hacks for other boards

• 1.7.1 (2023-06-15)
  • This is a maintenance release, to update the list of supported boards,
    and update the documentation.
    • No functional change.
  • Add MemoryBenchmark to collect flash and
    static memory usage.
    • For consistency with my other arduino libraries.
    • Also enables an automated script to verify that AUnit compiles under
      various microcontrollers.
  • Update supported boards and tiers
    • Update Flash.h to work better with Teensyduino,
      Adafruit SAMD, and Seeeduino SAMD boards by hacking around their bugs
      and inconsistent APIs
    • Add SAMD21 and SAMD51 boards to Tier 1
      • Add 2 SAMD boards from 2 different companies, to test their
        Arduino Core software:
        • Seeeduino XIAO M0 (SAMD21 48MHz ARM Cortex-M0+)
        • Adafruit ItsyBitsy M4 (SAMD51 120MHz ARM Cortex-M4)
      • SAMD21 and SAMD51 boards are back in Tier 1, as long as they use
        the traditional Arduino API instead of the new
        ArduinoCore-API.
      • Fortunately most third party SAMD21 and SAMD51 boards continue to
        use the traditional Arduino API.
    • Move Teensy 3.2 to Tier 2 ("Should work but not tested often")
      • This board is entering end-of-life.
      • As well, the Teensyduino environment integrates with the Arduino
        IDE and CLI in a way that's different than all other third-party
        Arduino boards. Some of my automation scripts do not work with
        Teensyduino, so it becomes very time consuming to test the Teensy
        boards.
      • All Teensy boards are now in Tier 2.
    • The various tiers are documented in the
      README.md.
  • Upgrade tool chain.
    • Arduino CLI from 0.19.2 to 0.33.0
    • Arduino AVR Core from 1.8.4 to 1.8.6
    • STM32duino from 2.2.0 to 2.5.0
    • ESP32 Core from 2.0.2 to 2.0.9
    • Teensyduino from 1.56 to 1.57 (unable to install 1.58)