Separate Storage of Value and Error in `Result` Class

[threeal]

Modify the Result class to store the value and error separately, enabling extraction using structured binding declaration as shown in the following example:

result::Result<Image> load_image(const std::filesystem::path& path);

int main() {
  const auto [image, error] = load_image(image_path);
  if (error) {
    // Handle the error case.
  } else {
    // Do something with the image.
  }
}