Migrate to ArduinoCore-API

Migrating / porting Arduino Core for ESP8266 to use https://github.com/arduino/ArduinoCore-API should bring closure to debates about Arduino compatibility.

According to the README there, at the time of entering this issue, in excerpts:
```
All Arduino official cores are being ported to the new structure so they take advantage of this single repo.

[...]
Some cores have been ported to the new structure, for example:

megaAVR (https://github.com/arduino/ArduinoCore-megaAVR)
nRF52-mbedos (https://github.com/arduino/ArduinoCore-nRF528x-mbedos)
classic AVR (https://github.com/arduino/ArduinoCore-avr/tree/api)
SAMD (https://github.com/arduino/ArduinoCore-samd/tree/api)
```