Function to_string

include/fmt/format.h:903–908

template <size_t SIZE>
FMT_NODISCARD auto to_string(const basic_memory_buffer<char, SIZE>& buf)
    -> std::string {
  auto size = buf.size();
  detail::assume(size < std::string().max_size());
  return {buf.data(), size};
}

// A writer to a buffered stream. It doesn't own the underlying stream.
class writer {