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/0x05_servo_rust/src/servo.rs

@@ -124,6 +124,7 @@ pub fn calc_clk_div(sys_hz: u32, servo_hz: f32, wrap: u32) -> f32 {
 
 #[cfg(test)]
 mod tests {
+    // Import all parent module items
     use super::*;
 
     #[test]