Function write_cp_utf16

include/boost/parser/detail/text/transcode_iterator.hpp:82–94

        template<typename OutIter>
        constexpr OutIter write_cp_utf16(char32_t cp, OutIter out)
        {
            if (cp < 0x10000) {
                *out = static_cast<char16_t>(cp);
                ++out;
            } else {
                *out = static_cast<char16_t>(cp >> 10) + high_surrogate_base;
                ++out;
                *out = static_cast<char16_t>(cp & 0x3ff) + low_surrogate_base;
                ++out;
            }
            return out;
        }

        inline constexpr char32_t surrogates_to_cp(char16_t hi, char16_t lo)
        {