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/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;