Everything should be using the keypad module instead.
Note: there are several boards that still had gamepadshift enabled. I
did not contact their authors to make sure they already switched to
keypad in their code and documentation. We should probably wait with
merging this for their go ahead.
* atmel-samd: Remove ASF3. This will break builds.
* atmel-samd: Add ASF4 for the SAMD21 and SAMD51.
* Introduce the supervisor concept to facilitate porting.
The supervisor is the code which runs individual MicroPython VMs. By
splitting it out we make it more consistent and easier to find.
This also adds very basic SAMD21 and SAMD51 support using the
supervisor. Only the REPL currently works.
This begins the work for #178.
