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/stm32f4/src/bin/multiprio.rs

@@ -62,11 +62,11 @@ use defmt_rtt as _; // global logger
 use panic_probe as _;
 
 use cortex_m_rt::entry;
-use embassy::executor::{Executor, InterruptExecutor};
-use embassy::interrupt::InterruptExt;
 use embassy::time::{Duration, Instant, Timer};
 use embassy::util::Forever;
+use embassy_stm32::executor::{Executor, InterruptExecutor};
 use embassy_stm32::interrupt;
+use embassy_stm32::interrupt::InterruptExt;
 
 #[embassy::task]
 async fn run_high() {