Add section about handling interrupts

A question that often comes up in the bare metal course is how to handle interrupts. It would probably be worthwhile to add a few pages to the afternoon about this. This would need to cover not just how we set up the exception vector but how we share state between the main program and the exception handlers, talking in more depth about spinlocks, critical sections, and how we can implement our own safe wrapper for these using `Send` and `Sync`.