This introduces a skip_if module that can be used by tests to
determine when they should be skipped due to the environment.
Some tests have been split in order to have finer grained skip
control.
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.
on a per port basis.
Also enables generating docs from inline RST in C code. Simply omits all
lines except those that start with //|. Indentation after "//| " will be
preserved.
