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/gpio/mod.rs

@@ -0,0 +1,30 @@
+//! # GPIO module
+//!
+//! The implementation of this GPIO module is heavily based on the
+//! [ATSAMD HAL implementation](https://docs.rs/atsamd-hal/0.13.0/atsamd_hal/gpio/v2/index.html).
+//!
+//! This API provides two different submodules, [`pin`] and [`dynpin`],
+//! representing two different ways to handle GPIO pins. The default, [`pin`],
+//! is a type-level API that tracks the state of each pin at compile-time. The
+//! alternative, [`dynpin`] is a type-erased, value-level API that tracks the
+//! state of each pin at run-time.
+//!
+//! The type-level API is strongly preferred. By representing the state of each
+//! pin within the type system, the compiler can detect logic errors at
+//! compile-time. Furthermore, the type-level API has absolutely zero run-time
+//! cost.
+//!
+//! If needed, [`dynpin`] can be used to erase the type-level differences
+//! between pins. However, by doing so, pins must now be tracked at run-time,
+//! and each pin has a non-zero memory footprint.
+//!
+//! ## Examples
+//!
+//! - [Blinky example](https://github.com/robamu-org/va108xx-hal-rs/blob/main/examples/blinky.rs)
+pub mod dynpins;
+pub use dynpins::*;
+
+pub mod pins;
+pub use pins::*;
+
+mod reg;