Pitch: `UnkeyedDecodingContainer.moveNext()` to skip items in deserialization

Evolution Pitches
17

About the best way to implement this, I am thinking about

  1. Adding mutating func skip() throws to the UnkeyedDecodingContainer protocol in Codable.swift.yb

  2. Adding and extension to the UnkeyedDecodingContainer protocol in Codable.swift.gyb doing the same thing I currently do, decoding an empty struct

// Default implementation of skip() in terms of decoding an empty struct
struct Empty: Decodable { }

extension UnkeyedDecodingContainer {
  public mutating func skip() throws {
    _ = try decode(Empty.self)
  }
}

The empty struct does not seem very elegant but this should be a reasonable default implementation that should work for the JSONDecoder and some other decoders.

Any custom decoder can then implement the method in specific way as needed or maybe throw a fatalerror if this method is really not suitable for it.