Create optional Python interrupt callback about canhack

[kentindell]

New firmware released (including new source).

[kentindell]

Upcoming release of firmware now has a receive callback function. Function is associated when the CAN controller class is created. It is passed a CANFrame instance, which it can act on.

Caveats:

1. MicroPython ISRs in Python are highly constrained (no heap, for example)
2. The CANFrame instance has a lifetime for the duration of the callback only: it must extract the data it needs from the instance and must not pass out a reference to the instance.

Remember that the callback is running from inside the ISR and if it executes for too long then this will disrupt CAN frame reception.

[beyonlo]

@kentindell That's a great news!

This will be a amazing new feature for this project :)

Thank you!