Implement CPU halting functionality

[marcluque]

As of now, the CPU cannot be halted. The function cpu_step does not have any consideration for this yet. This issue is supposed to be used as a reminder that the functionality needs to be implemented.

[mormod]

I commented on the review I made regarding this. I do not think that this is really necessary. The only instructions, which inhibit further execution are HALT and STOP, both of which are recoverable and do not initiate a shutdown of the system.

It could be that I overlooked something, but generally speaking there is no way cpu_step could fail (directly).