message: Make uint256 Span-convertible by adding ::data()
diff: @@ -56,6 +56,9 @@ public:
  void SetHex(const std::string& str);
  std::string ToString() const;
  + const unsigned char* data() const { return m_data; }
  + unsigned char* data() { return m_data; }
  +
  unsigned char* begin()
  {
  return &m_data[0];
  