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/0x07_i2c_rust/src/i2c.rs

@@ -116,6 +116,7 @@ pub fn format_scan_entry(buf: &mut [u8], addr: u8, found: bool) -> usize {
 
 #[cfg(test)]
 mod tests {
+    // Import all parent module items
     use super::*;
 
     #[test]