Also, rename Direction.IN to Direction.INPUT and Direction.OUT to Direction.OUTPUT.
This simplifies using them. Prior to the nativeio split this would have led to
clutter in the top-level namespace but having digitalio prevents this.
Fixes#152
This was done to allow greatly granularity when deciding what functionality
is built into each board's build. For example, this way pulseio can be
omitted to allow for something else such as touchio.
