Fix interrupt handling & add interrupt-based keyboard handling

[winstonallo]

We finally have interrupt-based keyboard handling 💀

Fixes:

• Properly return from interrupt handler by using the iretd instruction (instead of iret, which is meant for 16-bit operand sizes.

Features:

• Introduce interrupt-based keyboard handling (/src/keyboard.rs)
• Add x86::{exception,irq} modules
  • irq allows installing/uninstalling handlers at runtime (irq::install_handler(), irq::uninstall_handler()
• Add interrupt locks that can be used to either globally lock interrupts, or lock a single IRQ