Method instance

unittests/catch.hpp:9519–9523

!TBD: put in TLS

        // !TBD: put in TLS
        static auto instance() -> StringStreams& {
            if( !s_instance )
                s_instance = new StringStreams();
            return *s_instance;
        }
        static void cleanup() {
            delete s_instance;
            s_instance = nullptr;