Derive Serialize/Deserialize for certain simple structs

[ec2]

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This PR introduces "non-controversial" derives of Serialize and Deserialize for some structs that we persist to storage in our memory wallet implementation. This is by no means all the structs that we want to be able to serialize, but they are the ones that should be stable or are simple internal representations.

[ec2]

@nuttycom w.r.t. Proposal and associated structs, we need it to be serializable forwasm_bindgen so that we can pass it between JS and Wasm.

[ec2]

As per @daira 's suggestion, we will try using the protobuf encoding to do the passing back and forth.

[nuttycom]

I believe that the objectives of this PR were eventually satisfied by #1634