The reference to the upper USBDevice class is passed on
the EPHandler constructor.

this is the proper type to encapsulate a byte when data is available
or -1 when there is no data.

Port of https://github.com/arduino/Arduino/pull/4718 to SAMD core.

Fix #149

Port of https://github.com/arduino/Arduino/pull/4996

Previous 0 was always returned.

Change digitalPinToInterrupt to return pin, and use g_APinDescription[pin].ulExtInt to map pin inside attachInterrupt/detachInterrupt

This brings things inline with the atachInterrupt documentation for
usage:
attachInterrupt(digitalPinToInterrupt(pin), ISR, mode);