wire: add previous revealed secrets hashes to RS message

For proper blaming behavior, peers who publish their secrets are blamed and
removed from the following run if all secrets were revealed, but no other
misbehavior was detected.

In order to correctly pin the blame on the peers who disrupted the mix by
initially revealing their secrets messages, this blaming will only be
triggered when a reveal secrets message is received that does not reference
any other received secrets messages.

Because the secrets message hash now commits to these previous hashes by its
signature, an additional method is added to return the commitment hash (to be
published and verified in key exchange messages) that does not hash the
previous messages (as they do not exist at the time of creating the key
exchange).

While here, an issue was discovered and corrected in the serialization of the
MixVect type.  When the vector has zero length, deserializing would return
early after reading the first 0 dimension, but serialization was writing both
the count and message size dimensions.  This was corrected by changing the
serialization method to return early if the count is zero.