* 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.
`microcontroller.cpu.temperature` uses different ADC settings, and caused
AnalogIn to give wrong answers. AnalogIn can no longer assume it's the
only user of the ADC.
* Add microcontroller.cpu, the sole instance of microcontroller.Processor.
microcontroller.cpu.frequency is the clock frequency, in Hz.
microcontroller.cpu.temperature is the reading from the internal temperature sensor, in Celsius. None if not available.
* Squeeze firmware size by using -finline-limit. Otherwise non-Express builds were slightly too big.
* Update submodules.
* Fix documentation glitches
