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?

api/USBAPI.h から始め、issue で指定されている avr、samd、megaavr、mbed の core パスにある実装を、USBCore.cpp、USBDeviceClass、PluggableUSBModule を含めて比較します。issue では具体的な変更が定義されていないため、まず意図する互換性の範囲を確定してください。完了条件は、対象となる core 間で共通の USB API に合意することです。

索引モデルが issue の本文から書いたものです。