`Input` does not implement `RtcPin`

[dimpolo]

I'd like to express something like:

let mut pin = Input::new(io.pins.gpio4, Pull::None);
// use pin normally here

let ext0 = Ext0WakeupSource::new(&mut pin, WakeupLevel::High);
// use pin as a deep sleep wakeup source

This was possible before #1542.

We could either implement RtcPin for the wrapper structs or add a method to the wrapper structs that recovers the underlying Gpio struct