Today unfortunately was not a very fruitful day. I ended up spending way too much time on a code refactoring process in order to make the CALL instruction work. It was somewhat frustrating, because I had to change so much that worked perfectly up until now. Nonetheless, I will continue tomorrow and hopefully get the CALL Opcode working.
Among other things, the main refactoring was done to the EVM class itself: I now have a separate constructor and a start() function, which calls the run() handler with an instance of MachineState. So now I can create special opcode handlers for CALL and CREATE that will have the EVM class itself as parameter instead of only having access to the machine state. This is useful because during a CALL you are basically creating a new interpreter execution with the code of another contract and then return to the original execution.