Reduce the memory footprint of a decryption share in HoneyBadger

[vkomenda]

PR #68 introduced a new kind of HoneyBadger message that contains a decryption share of the CommonCoin for the threshold encryption scheme. The decryption share contains a vector of 48 bytes which is the compressed representation of a group element. #68 took a rather relaxed view on optimization of memory usage and simply cloned the same share from local storage to a message. This can be optimised by wrapping the locally stored share into an Rc and cloning that Rc to the message instead of the whole vector. However, since serialization is not derivable for Rc, that required defining custom Serialization and Deserialization instances for HoneyBadger::Message, or possibly using an attribute #[serde(with = "a_new_rc_serde_module")].

[afck]

Sorry, that was my fault! I somehow thought of the decryption share as containing a part of the transaction data (like the shares in Broadcast)!
Thanks for clearing that up! I'd say cloning 48 bytes is totally fine.