- 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`.
These files end up containing chip-specific info, so they deduplicate
really badly when building docs for all 1200 chips.
The doc generator removes files starting with `_`.