* 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.
Docs are here: http://tannewt-micropython.readthedocs.io/en/microcontroller/
It differs from upstream's machine in the following ways:
* Python API is identical across ports due to code structure. (Lives in shared-bindings)
* Focuses on abstracting common functionality (AnalogIn) and not representing structure (ADC).
* Documentation lives with code making it easy to ensure they match.
* Pin is split into references (board.D13 and microcontroller.pin.PA17) and functionality (DigitalInOut).
* All nativeio classes claim underlying hardware resources when inited on construction, support Context Managers (aka with statements) and have deinit methods which release the claimed hardware.
* All constructors take pin references rather than peripheral ids. Its up to the implementation to find hardware or throw and exception.
The tick timer needed to be reworked because the ASF delay functions also
use the SysTick timer. Now, it uses TC5 and calls out to the autoreset
logic every tick. Fixes#43.
Added neopixel status colors and corrected the latch time from ms to us.
Fixes#42.
