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-cortex-m/src/lib.rs

@@ -0,0 +1,8 @@
+#![no_std]
+
+// This mod MUST go first, so that the others see its macros.
+pub(crate) mod fmt;
+
+pub mod executor;
+pub mod interrupt;
+pub mod peripheral;