bincode deserialization of BTreeMap issues

[oxarbitrage]

In #114 we automatically serialize/deserialize the BTreeMaps introduced by the messages spec (https://github.com/ZcashFoundation/redjubjub/blob/fa74969d5653c620e831c8c51c096f051527d958/rfcs/0001-messages.md#payloads)

We found 2 issues with this during implementation that made us consider a manual implementation of serialize/deserialize for BTreeMaps or change the spec to match:

• Validation rules for duplicates and order in the map are silently fixed by bincode deserialization (Implement the messages spec #114 (comment)). This could be a problem if we want to throw an error instead.
• Each BTreeMap stores the length of itself in an u64 field when serialized using bincode. Spec says this should be an u8 (Implement the messages spec #114 (comment)).