Invincible Interruptor

There's still no userspace yet, but hey, we can now handle all sorts of traps and interrupts:

• Load page faults
• Store/AMO page faults
• Timer interrupts
• External interrupts (UART)

In particular, with external interrupts enabled, we can finally listen to our UART and respond to keyboard input. At the moment, it doesn't do much beyond echoing the input back to the user and powering down the board when it receives a Ctrl-C, but it's a good start nonetheless. Hopefully, with process management next on the list, we can start to see some semblance of a userspace.