CodableCBOREncoder: save order for encoding Codable objects

[Jnis]

I must send CBOR with a specially ordered data. But current implementation shuffle they everytime.
This fix allow to save order for encoding Codable objects.

ex.:

struct MyData1: Codable {
    let a: Int
    let b: Int
    let c: Int
}

// custom keys and order example
struct MyData2: Codable {
    let a: Int
    let b: Int
    let c: Int
    
    enum CodingKeys: Int, CodingKey {
        case c = 3
        case b = 2
        case a = 1
    }
}

let myData1 = MyData1(a: 1, b: 2, c: 3)
let myData2 = MyData2(a: 1, b: 2, c: 3)
let encoder = CodableCBOREncoder()
let result1 = try! encoder.encode(myData1) // a3 61 61 01 61 62 02 61 63 03
let result2 = try! encoder.encode(myData2) // a3 03 03 02 02 01 01 

[hamchapman]

Thanks for the PR!

We won't want to merge this as-is because it's imposing quite a large cost on users who don't care about the ordering. Every time a new item is added to a keyed container it has to iterate over the entirety of the existing storage for the container to check if there's already an equivalent key in there, which isn't efficient.

For something like this to be merged I think a couple of things would need to change:

1. the behaviour should be opt-in rather than the default
2. the implementation should probably use something like an OrderedDictionary internally, or perhaps you'd just need a partial version of that and do something more minimal just using an OrderedSet