~maddie/custom-processor-simulator

ref: 86418b49a48e7555a9243024dd42a94abc36a7b7 custom-processor-simulator/src d---------
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.