message: Add less for checksum256 and uint256
diff: */
  #pragma once
  #include <eosiolib/types.h>
  +#include <functional>
  +#include <tuple>
  namespace eosio {
  @@ -90,3 +92,27 @@ namespace eosio {
  /// @}
  } // namespace eos
  +
  +namespace std {
  + /**
  + * @brief provide less for uint256
  + */
  + template<>
  + struct less<uint256> : binary_function<uint256, uint256, bool> {
  + bool operator()( const uint256& lhs, const uint256& rhs ) const {
  + return std::tie(lhs.words[0], lhs.words[1], lhs.words[2], lhs.words[3]) <
  + std::tie(rhs.words[0], rhs.words[1], rhs.words[2], rhs.words[3]);
  + }
  + };
  +
  + /**
  + * @brief provide less for checksum256
  + */
  + template<>
  + struct less<checksum256> : binary_function<checksum256, checksum256, bool> {
  + bool operator()( const checksum256& lhs, const checksum256& rhs ) const {
  + return memcmp(&lhs, &rhs, sizeof(lhs)) < 0;
  + }
  + };
  +
  +} // namespace std
  