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/dma/dma.rs

@@ -1,7 +1,7 @@
 use core::sync::atomic::{fence, Ordering};
 use core::task::Waker;
 
-use embassy::interrupt::{Interrupt, InterruptExt};
+use crate::interrupt::{Interrupt, InterruptExt};
 use embassy::waitqueue::AtomicWaker;
 
 use crate::_generated::DMA_CHANNEL_COUNT;