Add embassy-cortex-m crate.

- Move Interrupt and InterruptExecutor from `embassy` to `embassy-cortex-m`.
- Move Unborrow from `embassy` to `embassy-hal-common` (nothing in `embassy` requires it anymore)
- Move PeripheralMutex from `embassy-hal-common` to `embassy-cortex-m`.

examples/nrf/src/bin/multiprio.rs

@@ -59,11 +59,11 @@
 
 use cortex_m_rt::entry;
 use defmt::{info, unwrap};
-use embassy::executor::{Executor, InterruptExecutor};
-use embassy::interrupt::InterruptExt;
 use embassy::time::{Duration, Instant, Timer};
 use embassy::util::Forever;
+use embassy_nrf::executor::{Executor, InterruptExecutor};
 use embassy_nrf::interrupt;
+use embassy_nrf::interrupt::InterruptExt;
 
 use defmt_rtt as _; // global logger
 use panic_probe as _;

examples/nrf/src/bin/usb_hid_keyboard.rs

@@ -8,11 +8,11 @@ use core::sync::atomic::{AtomicBool, Ordering};
 use defmt::*;
 use embassy::channel::Signal;
 use embassy::executor::Spawner;
-use embassy::interrupt::InterruptExt;
 use embassy::time::Duration;
 use embassy::util::{select, select3, Either, Either3};
 use embassy_nrf::gpio::{Input, Pin, Pull};
 use embassy_nrf::interrupt;
+use embassy_nrf::interrupt::InterruptExt;
 use embassy_nrf::pac;
 use embassy_nrf::usb::Driver;
 use embassy_nrf::Peripherals;