[feature request] implement StatefulOutputPin for GPIO output pins

[Jorge Aparicio]

It would be nice if pins like PB13 implemented this trait so we can read the current output level (e.g. via the is_set_low method).

https://docs.rs/embedded-hal/0.2.4/embedded_hal/digital/v2/trait.StatefulOutputPin.html

(a ToggleableOutputPin implementation would be nice too)