Refactored GPIO module

- The GPIO module uses type-level programming now
- Implementation heavily based on the ATSAMD GPIO HAL:
  https://docs.rs/atsamd-hal/0.13.0/atsamd_hal/gpio/v2/index.html
- Changes to API, but no passing of peripheral references necessary
  anymore. All examples and tests updated accordingly

src/prelude.rs

@@ -1,8 +1,8 @@
 //! Prelude
 pub use embedded_hal::prelude::*;
 
-pub use crate::gpio::GpioExt as _va108xx_hal_gpio_GpioExt;
-
+// embedded-hal doesn’t yet have v2 in its prelude, so we need to
+// export it ourselves
 pub use embedded_hal::digital::v2::InputPin as _embedded_hal_gpio_InputPin;
 pub use embedded_hal::digital::v2::OutputPin as _embedded_hal_gpio_OutputPin;
 pub use embedded_hal::digital::v2::StatefulOutputPin as _embedded_hal_gpio_StatefulOutputPin;