firmware: Make modules public to enable documentation warnings.

hackamp/firmware/src/halext/mod.rs

+//! Hardware drivers missing in the HAL.
 #![warn(missing_docs)]

hackamp/firmware/src/main.rs

 use audio_buffer::AudioBuffer;
 use pins::Pins;
 
-mod audio_buffer;
-mod halext;
-mod mixer;
-mod pins;
+// Remove "pub" to check whether there is any dead code.
+pub mod audio_buffer;
+pub mod halext;
+pub mod mixer;
+pub mod pins;
 
 #[rtic::app(device = stm32f4xx_hal::stm32, peripherals = true)]
 const APP: () = {

hackamp/firmware/src/mixer.rs

+//! Functionality to mix and forward different audio streams.
 #![warn(missing_docs)]

hackamp/firmware/src/pins.rs

+//! GPIO pin configuration.
 #![warn(missing_docs)]
 
 /// Type which contains all GPIO pins.