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/crc/v1.rs

@@ -3,7 +3,7 @@ use core::marker::PhantomData;
 use crate::pac::CRC as PAC_CRC;
 use crate::peripherals::CRC;
 use crate::rcc::sealed::RccPeripheral;
-use embassy::util::Unborrow;
+use crate::Unborrow;
 use embassy_hal_common::unborrow;
 
 pub struct Crc<'d> {

embassy-stm32/src/crc/v2v3.rs

@@ -4,7 +4,7 @@ use crate::pac::crc::vals;
 use crate::pac::CRC as PAC_CRC;
 use crate::peripherals::CRC;
 use crate::rcc::sealed::RccPeripheral;
-use embassy::util::Unborrow;
+use crate::Unborrow;
 use embassy_hal_common::unborrow;
 
 pub struct Crc<'d> {