examples: uno: Forward port the uno-panic example

Show how to write a custom panic handler.  Update the implementation to
be more device-agnostic to make it easier to reuse the code with other
boards.

examples/arduino-uno/src/bin/uno-panic.rs

@@ -0,0 +1,59 @@
+//! Example of a custom panic handler.
+//!
+//! The panic handler will print out where the panic occurred and then blink the oboard LED rapidly
+//! to make the user aware of the problem.
+#![no_std]
+#![no_main]
+
+use arduino_hal::prelude::*;
+
+#[panic_handler]
+fn panic(info: &core::panic::PanicInfo) -> ! {
+    // disable interrupts - firmware has panicked so no ISRs should continue running
+    avr_device::interrupt::disable();
+
+    // get the peripherals so we can access serial and the LED.
+    //
+    // SAFETY: Because main() already has references to the peripherals this is an unsafe
+    // operation - but because no other code can run after the panic handler was called,
+    // we know it is okay.
+    let dp = unsafe { arduino_hal::Peripherals::steal() };
+    let pins = arduino_hal::pins!(dp);
+    let mut serial = arduino_hal::default_serial!(dp, pins, 57600);
+
+    // Print out panic location
+    ufmt::uwriteln!(&mut serial, "Firmware panic!\r").void_unwrap();
+    if let Some(loc) = info.location() {
+        ufmt::uwriteln!(
+            &mut serial,
+            "  At {}:{}:{}\r",
+            loc.file(),
+            loc.line(),
+            loc.column(),
+        )
+        .void_unwrap();
+    }
+
+    // Blink LED rapidly
+    let mut led = pins.d13.into_output();
+    loop {
+        led.toggle();
+        arduino_hal::delay_ms(100);
+    }
+}
+
+#[arduino_hal::entry]
+fn main() -> ! {
+    let dp = arduino_hal::Peripherals::take().unwrap();
+    let pins = arduino_hal::pins!(dp);
+    let mut serial = arduino_hal::default_serial!(dp, pins, 57600);
+
+    ufmt::uwriteln!(&mut serial, "Hello from Arduino!\r").void_unwrap();
+    ufmt::uwriteln!(&mut serial, "Panic in 5 seconds!\r").void_unwrap();
+
+    arduino_hal::delay_ms(5000);
+
+    // Panic messages cannot yet be captured because they rely on core::fmt
+    // which is way too big for AVR
+    panic!();
+}