Add serialization and deserialization to Coins

[shanev]

This allows Coins to be used in storage without depending on Vec<Coin> which could sort in non-deterministic ways.

[webmaster128]

depending on Vec which could sort in non-deterministic ways

Using to_vec and into_vec actually does guarantee sorting. @chipshort could you ensure this property is tested and documented?

I'm not sure if it is good to have two different ways of serializing coins. The proposed change will probably create a JSON dictionary and expose internal structure. However, this structure is subject to change. My gut feeling is that if we implement Serialize/Deserialize for Coins, we should implement it explicitly via to_vec.

[webmaster128]

As of now, I consider .to_vec()/.into_vec() the right (tm) way to do this. See also #1821. This requires coins: Vec<Coin> in the state object and you need a Coins::try_from(state.coins)? which might be slightly inconvenient but should do the job. Feel free to re-open if there is anything left here.