Add a check() overload for lazy assertion message generation via callable

[heifner]

See AntelopeIO/cdt#387

Example:

      eosio::check(eosio::current_time_point() >= time, [&] {
         std::string err_msg = "Upgrade failed: Account '";
         err_msg += account.to_string();
         err_msg += "' disabled until ";
         err_msg += time.to_string();
         return err_msg;
      });