Change `VerifierChannel:read_trace_commitments()` to return `(H::Digest, Option<H::Digest>)`

[Al-Kindi-0]

Probably not for this PR, but we should change VerifierChannel:read_trace_commitments() to return (H::Digest, Option<H::Digest>) rather than &[H::Digest]. Then, line 172 above could be:

let (main_trace_commitments, aux_trace_commitment) = channel.read_trace_commitments();

Originally posted by @irakliyk in #359 (comment)