Convert existing cores

I'm opening up this issue to track official cores that have or still need to be converted to use the new API code (replacing https://github.com/arduino/Arduino/issues/5771#issuecomment-533204022)

Done:
* megaAVR (https://github.com/arduino/ArduinoCore-megaAVR)
* nRF52-mbedos (https://github.com/arduino/ArduinoCore-nRF528x-mbedos)

In progress:
* classic AVR (https://github.com/arduino/ArduinoCore-avr/tree/api)
* SAMD (https://github.com/arduino/ArduinoCore-samd/tree/api)

TODO:
* SAM(https://github.com/arduino/ArduinoCore-sam)
* ARC32 (https://github.com/arduino/ArduinoCore-arc32)

Review the new API code in ArduinoCore-API and compare the completed megaAVR and nRF52-mbedos conversions with the TODO SAM and ARC32 core repositories. Check the classic AVR and SAMD api branches for context on the ongoing migration. Done means the official SAM and ARC32 cores have been converted to use the new API.

Written by the indexing model from the issue text.