Function for_each_codepoint

include/fmt/format.h:621–653

// corresponding to the code point. cp is invalid_code_point on error.
template <typename F>
FMT_CONSTEXPR void for_each_codepoint(string_view s, F f) {
  auto decode = [f](const char* buf_ptr, const char* ptr) {
    auto cp = uint32_t();
    auto error = 0;
    auto end = utf8_decode(buf_ptr, &cp, &error);
    bool result = f(error ? invalid_code_point : cp,
                    string_view(ptr, error ? 1 : to_unsigned(end - buf_ptr)));
    return result ? (error ? buf_ptr + 1 : end) : nullptr;
  };

  auto p = s.data();
  const size_t block_size = 4;  // utf8_decode always reads blocks of 4 chars.
  if (s.size() >= block_size) {
    for (auto end = p + s.size() - block_size + 1; p < end;) {
      p = decode(p, p);
      if (!p) return;
    }
  }
  auto num_chars_left = to_unsigned(s.data() + s.size() - p);
  if (num_chars_left == 0) return;

  // Suppress bogus -Wstringop-overflow.
  if (FMT_GCC_VERSION) num_chars_left &= 3;
  char buf[2 * block_size - 1] = {};
  copy<char>(p, p + num_chars_left, buf);
  const char* buf_ptr = buf;
  do {
    auto end = decode(buf_ptr, p);
    if (!end) return;
    p += end - buf_ptr;
    buf_ptr = end;
  } while (buf_ptr < buf + num_chars_left);
}

FMT_CONSTEXPR inline auto display_width_of(uint32_t cp) noexcept -> size_t {
  return to_unsigned(