Implement the rest of the arithmetic ops.
This implements subtraction, multiplication, and division, as well as the associated
checks for overflows and div by 0

Implemented addition and the generalised parts of handling arithmetic ops.
In particular, implemented the invert (I) flag, and ensured that the program
counter is unwritable for arithmetic instructions.

Initial commit
Creates processor.rs as well as the processor struct and the start of the code for executing instructions.