String trim operation with C++20 views.

trim_cpp20.hpp

inline constexpr auto trim_front = views::drop_while(::isspace);
inline constexpr auto trim_back = views::reverse
  | views::drop_while(::isspace)
  | views::reverse;

inline constexpr auto trim = trim_front | trim_back;

std::string trim_str(const std::string& str) {
  return str | trim | rangesnext::to<std::string>;
}