806: Add embassy-cortex-m crate. r=Dirbaio a=Dirbaio
- 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`.
Co-authored-by: Dario Nieuwenhuis <dirbaio@dirbaio.net>