in Sources/TwitterApacheThrift/ThriftDecoder.swift [191:197]
func decodeKey<T>(_ type: T.Type) throws -> T where T : Decodable {
guard case .keyedCollection(let collection) = decoder.value else {
throw ThriftDecoderError.keyOrderMismatch
}
let key = collection.value[currentIndex].key
return try decoder.decodeType(type: type, value: key)
}