Odds and ends

- Made all functions snake_case for consistency
- Fixed the backspace key (actually removes the character from the screen now)
- Caught the invalid opcode instruction and added debugging output to it
- More debugging output at boot a bit clearer.

Nothing major. The screen printing is a bit weird when you try and execute IRB and the ISR6 handler gets run. Not sure why we're getting an invalid opcode error, though?