SCALE-native JSON value

Context

Continuation of:

4900
5012

In short, Iroha supports arbitrary JSON payloads for certain actions. In SCALE they are encoded as JSON strings (JsonString type), while in JSON format they are inlined as values.

This is a proposal of an alternative solution: introduce as SCALE-native JsonValue type, encoded not as a string, but directly as an enum:

enum JsonValue {
    Null,
    Bool(bool),
    Number(Number),
    String(String),
    Array(Vec<JsonValue>),
    Object(Map<String, JsonValue>),
}

_this is a literal copy of serde_json::Value_

Benefits

Compact encoding of JSON - no need to encode "[{}], characters & whitespace
Possibly faster deserialisation - needs benchmarking and careful implementation
Consistency in the schema - JsonValue represents f

Downsides

Needs custom implementation

Why it is String now

AFAIK the initial choice of string representation for JSON values is an efficiency concern: we don't want to pay for extra (de)serialization while transmitting data to/from WASM smartcontracts.

Basically, string was used as a way to have lazy encoding of data, as requested.

I think it should be possible to achieve lazy encoding in a more direct way with some sort of Lazy<T> container, making it both more explicit and flexible.

hyperledger / iroha