Adds a skeleton framework for CIO ("Common I/O") feature cards. The
first feature card to be implemented will be the "PORTS" serial MUX.
Part of this support involved reworking IRQ handling in the CPU. It
now respects both IRQ Vector and IPL.
This change also removes all 'assert(0)' calls from the simulator and
replaces them with generic "Simulator Error" halts. These should only
happen if there's a genuine logic error lurking somewhere.
Clock reset was not being handled gracefully at all, leading some
diagnostic tests not to pass, or not to pass consistently.
This change will "kick" the system clock whenever the timer divider is
reset, so the new divider is picked up immediately.
