Trace sorting: Move add gate out of aux block

[ledwards2225]

Execution trace sorting requires the addition of dummy gates where they were previously not required in cases where one gate was reading into the next row of different gate type. Doing this in all necessary locations causes the double_verify_proof circuit to go beyond the 2^19 limit imposed by WASM + plonk. I'm avoiding one such dummy gate by simply allowing an add gate to be placed into the aux block. (This isn't a long term solution because it requires that both the aux and arithmetic relations run on the entire aux block). Once plonk goes away, this can be removed and replaced with an add gate and a dummy gate, both placed in the arithmetic block.