Decide ExtractFinal circuit output

[lonerapier]

decide how we want to do the final value extraction. Right now I wrote a new output signal value. I think we may want to have the step_out all zeroed out and perhaps even set the curr_depth signal (step_out[-1]) to be something like -1 (bn254::max). This means no circuit can ever meaningfully placed after this one. This gives us assurance no funny business happens after parsing and we can even assert that a ROM must end with this circuit.

Originally posted by @Autoparallel in #96 (comment)