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`.

embassy-stm32/src/timer/mod.rs

@@ -1,4 +1,4 @@
-use embassy::interrupt::Interrupt;
+use crate::interrupt::Interrupt;
 
 use crate::rcc::{sealed::RccPeripheral as __RccPeri, RccPeripheral};
 use crate::time::Hertz;