sstring: declare nested type with typename #191

Job	Run time
Test with DPDK enabled / test	23m 6s
Test with C++20 modules enabled / test	11m 53s
Test (clang++, C++20, debug) / test	40m 10s
Test (clang++, C++20, dev) / test	16m 7s
Test (clang++, C++20, release) / test	22m 11s
Test (clang++, C++23, debug) / test	17m 4s
Test (clang++, C++23, dev) / test	32m 34s
Test (clang++, C++23, release) / test	19m 39s
Test (g++, C++20, debug) / test	19m 20s
Test (g++, C++20, dev) / test	40m 3s
Test (g++, C++20, release) / test	16m 7s
Test (g++, C++23, debug) / test	19m 20s
Test (g++, C++23, dev) / test	12m 36s
Test (g++, C++23, release) / test	17m 5s
	5h 7m 15s

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