Program ROM bus constraints

[morganthomas]

Currently the program ROM bus channel sends are commented out (in CpuChip::global_sends, in cpu/src/lib.rs). As a result, the consistency of the execution trace with the program ROM contents is not being checked. Re-enable these sends, causing the consistency of the execution trace with the program ROM contents to be enforced. Make sure that the prover still runs and the verifier still accepts the proofs output by the prover after this change.