Proof of concept: Two types of decoders

[hello-im-szymon]

This allows for a bit less strict decoders. For example:

private struct ImageDecoder: ConcreteTypeDecoder {

    enum DecodingError: Error {
        case invalidData
    }

    func decode(_ data: Data) throws -> UIImage {
        guard let image = UIImage(data: data) else {
            throw DecodingError.invalidData
        }
        return image
    }

}

The API is not changed for existing clients.

[hello-im-szymon]

not sure about the naming