examples/nrf: add self_spawn example.

This serves as a compile-test of possible typecheck loops due to
TAIT shenanigans.

examples/nrf/src/bin/self_spawn.rs

@@ -0,0 +1,24 @@
+#![no_std]
+#![no_main]
+#![feature(type_alias_impl_trait)]
+
+use defmt::{info, unwrap};
+use embassy::executor::Spawner;
+use embassy::time::{Duration, Timer};
+use embassy_nrf::Peripherals;
+
+use defmt_rtt as _; // global logger
+use panic_probe as _;
+
+#[embassy::task(pool_size = 2)]
+async fn my_task(spawner: Spawner, n: u32) {
+    Timer::after(Duration::from_secs(1)).await;
+    info!("Spawning self! {}", n);
+    unwrap!(spawner.spawn(my_task(spawner, n + 1)));
+}
+
+#[embassy::main]
+async fn main(spawner: Spawner, _p: Peripherals) {
+    info!("Hello World!");
+    unwrap!(spawner.spawn(my_task(spawner, 0)));
+}