It was only useful for doing #[embassy_executor::main(config = "config()")]`. Now that
it's gone, it makes more sense to build the config in main directly.
- 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`.