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/lib.rs

@@ -7,10 +7,15 @@ pub mod gpio;
 pub mod prelude;
 pub mod time;
 pub mod timer;
+pub mod typelevel;
 pub mod uart;
 
 pub use va108xx as pac;
 
-mod sealed {
+mod private {
+    /// Super trait used to mark traits with an exhaustive set of
+    /// implementations
     pub trait Sealed {}
 }
+
+pub(crate) use private::Sealed;