Add // comments to all use statements in driver .rs files

- Add // comments above top-level use imports in uart.rs, blink.rs, button.rs
- Add // comments above test module use imports (use super::*, Infallible,
  ErrorType) in all 8 driver .rs files
- All 8 drivers build, all 75 tests pass

drivers/0x04_pwm_rust/src/pwm.rs

@@ -79,6 +79,7 @@ pub fn duty_to_level(percent: u8, wrap: u32) -> u32 {
 
 #[cfg(test)]
 mod tests {
+    // Import all parent module items
     use super::*;
 
     #[test]