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/typelevel.rs

@@ -0,0 +1,60 @@
+///! # Module support type-level programming
+///
+/// This module is more or less taken from the
+/// [ATSAMD HAL](https://docs.rs/atsamd-hal/0.13.0/atsamd_hal/typelevel/index.html).
+/// You can find a thorough explanation of the patterns used there.
+use crate::Sealed;
+
+/// Type-level version of the [None] variant
+#[derive(Default)]
+pub struct NoneT;
+impl Sealed for NoneT {}
+
+/// Marker trait for type identity
+///
+/// This trait is used as part of the [`AnyKind`] trait pattern. It represents
+/// the concept of type identity, because all implementors have
+/// `<Self as Is>::Type == Self`. When used as a trait bound with a specific
+/// type, it guarantees that the corresponding type parameter is exactly the
+/// specific type. Stated differently, it guarantees that `T == Specific` in
+/// the following example.
+///
+/// ```
+/// where T: Is<Type = Specific>
+/// ```
+///
+/// Moreover, the super traits guarantee that any instance of or reference to a
+/// type `T` can be converted into the `Specific` type.
+///
+/// ```
+/// fn example<T>(mut any: T)
+/// where
+///     T: Is<Type = Specific>,
+/// {
+///     let specific_mut: &mut Specific = any.as_mut();
+///     let specific_ref: &Specific = any.as_ref();
+///     let specific: Specific = any.into();
+/// }
+/// ```
+///
+/// [`AnyKind`]: #anykind-trait-pattern
+pub trait Is
+where
+    Self: Sealed,
+    Self: From<IsType<Self>>,
+    Self: Into<IsType<Self>>,
+    Self: AsRef<IsType<Self>>,
+    Self: AsMut<IsType<Self>>,
+{
+    type Type;
+}
+
+/// Type alias for [`Is::Type`]
+pub type IsType<T> = <T as Is>::Type;
+
+impl<T> Is for T
+where
+    T: Sealed + AsRef<T> + AsMut<T>,
+{
+    type Type = T;
+}