Makes Identifier RawRepresentable (explicitely)

[s4cha]

From what I can see, Identifier is already complying to the RawRepresentable protocol implicitly:
It has public let rawValue + public init(rawValue: Value.RawIdentifier).

This just adds this compliance explicitly.

For my use case this fixes the JSON parsing (using Arrow)

extension YPBreed: ArrowParsable {
    public mutating func deserialize(_ json: JSON) {
        let resource = resourceKeyFrom(json)
        id <-- resource["id"]
        [...]
    }
}

Prior, the parsing wouldn't work since it relies on RawRepresentable 👍