add some instructions at $2000 for convenient debug with simulation.
clean up some old files.
remove old source files from simulation command line.
fix bug in z serial monitor command.
fix bug due to making cpu state 16 bits wide.
begin implementing ADC and other accumulator operations.
try to fix PC advancement when CPU under serial monitor control.
normalise PC incrementing.
try to fix other places where PC gets incremented when it shouldnt
when exiting instructions if single stepping.