Method release_key

include/boost/json/detail/string_impl.hpp:225–236

        std::size_t capacity);

    char const*
    release_key(
        std::size_t& n) noexcept
    {
        BOOST_ASSERT(
            k_.k == key_string_);
        n = k_.n;
        auto const s = k_.s;
        // prevent deallocate
        k_.k = short_string_;
        return s;
    }

    void
    destroy(