Redesign SWM API

[hannobraun]

I'm unhappy with some aspects of the swm module's design:

• The distribution of responsibility between hand-written and generated code: Handle generated code consistenly #48
• The number of traits for fixed and movable functions. Once Handle generated code consistenly #48 has been implemented, I believe we can reduce this to one trait for fixed and one trait for movable functions.
• The relationship of AdcChannel/InputFunction/OutputFunction to the traits for fixed and movable functions. There are so many traits, and a lot of complexity when using them in function signatures. I don't have a clear vision here, yet, only the vague feeling that there must be a better way.
• The public API for the SWM lives in the gpio module, which is weird at best, confusing at worst. Once Handle generated code consistenly #48 is implemented, it can be moved to the new FixedFunction/MovableFunction types.
• The gpio and swm modules are entangled with each other. I think it would be less complex, if swm owned Pin and related types, and gpio was restricted to GPIO APIs.

[hannobraun]

Updated the original comment. Some items have been completed.

[hannobraun]

Updated original comment. Thanks to #90 and #91, the API has been radically simplified. There are only 2 methods total now (down from 8): One for assigning functions, one for unassigning them.

[hannobraun]

Updated original comment. #92 moved the function assignment API to the swm module. I also added another item about the entanglement between the gpio and swm modules.