Enforce 12-bit PC and I limits

[jon-axon]

These fields are 16-bit but for the original CHIP-8 interpreter they were 12-bit, so for ideal emulation in CHIP-8 mode we should throw an error if an attempt is made to assign to/increment these registers beyond the 12-bit max.

This will be most easily achieved by replacing direct assignment to the fields with a setter method that specifically checks for emulation mode and checks for 12-bit overflow if appropriate.

[jon-axon]

It seems this is actually misinformation, as per https://laurencescotford.com/chip-8-on-the-cosmac-vip-indexing-the-memory/

The PC and I registers were fully 16-bit on the COSMAC, despite the address space only being at most 12-bit addressable.

[jon-axon]

Closed as per the above.