Turn on blinkenlights from the PDP core

Implement switch features:
    Deposit - needs a state machine
        [pc++] = switch_sr
    Examine - needs a state machine
        led_memdata = [pc++]

Implement remaining TTY output instructions

Jules thinks clock is most likely interrupt source
    pdp8 source tries to map clock interrupts to "real time" - we might want to mangle that to make it deterministic

Sources of interrupts observed:
    PTR flag is set from loader, might be ignorable
    Memory management is poked from inside ISRs, might be ignorable (MMU should be optional)
    TTO ready for output interrupt - this looks like a very likely candidate
    TTI input ready interrupt - this looks like a very likely candidate
        Looks like these happen even when there is no input available - might be caused by the program itself doing something a little funny

Implement IF/DF fields
    Will need to update behaviour of switch_load_add

Add a macro for reversing polarity of switch_sing_step and switch_sing_inst and
making switch_dep happen on key lift instead of key press