Function ltrim

include/util/string.hpp:8–11

const std::string WHITESPACE = " \n\r\t\f\v";

inline std::string ltrim(const std::string& s) {
  size_t begin = s.find_first_not_of(WHITESPACE);
  return (begin == std::string::npos) ? "" : s.substr(begin);
}

inline std::string rtrim(const std::string& s) {
  size_t end = s.find_last_not_of(WHITESPACE);