removed problematic extern declarations; fixed analogInputToDigitalPin() macro

"extern" declarations for arrays defined in the pins_arduino.h file were
causing link errors when those symbols needed to be resolved. The
declarations just needed deleting.

Fixed analogInputToDigitalPin() macro. It was mapping e.g. A0 -> D21 and
A7 -> D14, which was incorrect. The correct mapping is A0 -> D14 ... A7
-> D21.