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/0x01_uart_rust/src/uart.rs

@@ -25,12 +25,19 @@
 //! OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 //! SOFTWARE.
 
+// Rate extension trait for .Hz() baud rate construction
 use fugit::RateExtU32;
+// Non-blocking I/O helper for UART read/write
 use nb::block;
+// Alias our HAL crate
 use rp235x_hal as hal;
+// Clock trait for accessing system clock frequency
 use hal::Clock;
+// UART configuration and peripheral types
 use hal::uart::{DataBits, Enabled, StopBits, UartConfig, UartPeripheral};
+// GPIO pin types and function selectors
 use hal::gpio::{FunctionNull, FunctionUart, Pin, PullDown, PullNone};
+// UART0 peripheral singleton type
 use hal::pac::UART0;
 
 /// Type alias for the configured TX pin (GPIO 0, UART function, no pull).
@@ -158,6 +165,7 @@ impl UartDriver {
 
 #[cfg(test)]
 mod tests {
+    // Import all parent module items
     use super::*;
 
     #[test]

drivers/0x02_blink_rust/src/blink.rs

@@ -25,6 +25,7 @@
 //! OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 //! SOFTWARE.
 
+// Embedded HAL digital output traits for GPIO pin control
 use embedded_hal::digital::{OutputPin, StatefulOutputPin};
 
 /// GPIO output / LED blink driver that owns a single output pin.
@@ -95,8 +96,11 @@ impl<P: OutputPin + StatefulOutputPin> BlinkDriver<P> {
 
 #[cfg(test)]
 mod tests {
+    // Import all parent module items
     use super::*;
+    // Infallible error type for mock pin implementations
     use core::convert::Infallible;
+    // Error type trait for mock pin implementations
     use embedded_hal::digital::ErrorType;
 
     struct MockPin {

drivers/0x03_button_rust/src/button.rs

@@ -25,6 +25,7 @@
 //! OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 //! SOFTWARE.
 
+// Embedded HAL digital I/O traits for GPIO pin control
 use embedded_hal::digital::{InputPin, OutputPin, StatefulOutputPin};
 
 /// Push-button GPIO input driver with debounce.
@@ -123,8 +124,11 @@ impl<L: OutputPin + StatefulOutputPin> ButtonLed<L> {
 
 #[cfg(test)]
 mod tests {
+    // Import all parent module items
     use super::*;
+    // Infallible error type for mock pin implementations
     use core::convert::Infallible;
+    // Error type trait for mock pin implementations
     use embedded_hal::digital::ErrorType;
 
     struct MockInputPin {

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]

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]

drivers/0x06_adc_rust/src/adc.rs

@@ -65,6 +65,7 @@ pub fn raw_to_celsius(raw: u16) -> f32 {
 
 #[cfg(test)]
 mod tests {
+    // Import all parent module items
     use super::*;
 
     #[test]

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]

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]