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-nrf/src/chips/nrf52820.rs

@@ -212,7 +212,7 @@ impl_ppi_channel!(PPI_CH31, 31 => static);
 
 pub mod irqs {
     use crate::pac::Interrupt as InterruptEnum;
-    use embassy_macros::interrupt_declare as declare;
+    use embassy_macros::cortex_m_interrupt_declare as declare;
 
     declare!(POWER_CLOCK);
     declare!(RADIO);