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/0x08_lcd1602_rust/src/lcd1602.rs

@@ -130,6 +130,7 @@ pub fn format_counter(buf: &mut [u8], count: u32) -> usize {
 
 #[cfg(test)]
 mod tests {
+    // Import all parent module items
     use super::*;
 
     #[test]