cpu: implement exceptions / IRQ:s

[martinlindhe]

https://wiki.osdev.org/IRQ

At the very least, the following IRQ:s should be implemented:

IRQ 0 - Programmable Interrupt Timer Interrupt (needed by src/pit.rs)
IRQ 1 - Keyboard Interrupt
IRQ 12 - PS2 Mouse

In order to do this, we need to:

• implement the Programmable Interrupt Controller in src/pic.rs
• implement the Interrupt Vector Table