Each core provides a different USBAPI

The Arduino Core API defines a very low level USB api in [api/USBAPI.h](https://github.com/arduino/ArduinoCore-API/blob/master/api/USBAPI.h). This API is implemented by the avr core in [cores/arduino/USBCore.cpp](https://github.com/arduino/ArduinoCore-avr/blob/master/cores/arduino/USBCore.cpp). But it is not implemented by samd core, which implements the same functionality but in its own [`USBDeviceClass` class](https://github.com/arduino/ArduinoCore-samd/blob/master/cores/arduino/USB/USBCore.cpp). The USB implementation in megaavr core (`USBCore.cpp`) is a copy of the avr usb implementation with some deviations. The mbed core implements its own version of the `PluggableUSBModule`.

As a result, each core implements its own USB API which is incompatible with the others, there exist many code duplicates and users of the USB API have to understand and adapt their code to each individual core implementation.

Are there any plans to improve this situation?

Inizia da api/USBAPI.h e confronta le implementazioni nei percorsi dei core avr, samd, megaavr e mbed indicati nell’issue, inclusi USBCore.cpp, USBDeviceClass e PluggableUSBModule. L’issue non definisce una modifica concreta, quindi stabilisci prima l’ambito di compatibilità previsto; il lavoro sarà completato quando sarà concordata una USB API comune tra i core interessati.

Scritto dal modello di indicizzazione a partire dal testo della issue.