Add PinInput and PinOutput HAL

[soypat]

machine.Pin is not compileable by upstream go. To promote development of drivers which are usable by upstream Go I propose we add the following API.

// PinInput is hardware abstraction for a pin which receives a
// digital signal and reads it (high or low voltage).
type PinInput func() (level bool)

// PinOutput is hardware abstraction for a pin which outputs a
// digital signal (high or low voltage).
type PinOutput func(level bool)

[deadprogram]

What about #749 ?

[soypat]

I missed that @deadprogram. Will comment