Add CustomStringConvertible to `SwiftyGPIO.RaspberryGPIO`

[Helge Heß]

It would be nice if SwiftyGPIO.RaspberryGPIO would have a description (aka conform to the CustomStringConvertible protocol) containing all its properties. When you currently do a

button.onChange  { gpio in
  print("Value changed:", gpio)
}

You just get a:

Value changed: SwiftyGPIO.RaspberryGPIO

[uraimo]

Agreed, I suspect it was printing something more useful in one of the older releases, hmmm.