SCT signature verification, encode/decode and accessor functions

Hmm, as I said, I think you want to be able to construct the SCT from scratch as well. Pointing directly to the serialized encoding seems brittle.

I wonder if caching the encoding is worth it. For certs, OpenSSL caches the encoding because
1) - some certs have broken non-DER for which re-encoding would break signature validation; and
2) performance.

Neither seems like a concern here, so just drop the "sct->sct"?