For union, add naive mutex-locked std::[unordered_]map alternative to… #808

Job	Run time
ubu-24 \| g++-14 \| c++2b \| libstdc++ \| ubuntu-24.04	6m 8s
ubu-24 \| g++-13 \| c++2b \| libstdc++ \| ubuntu-24.04	5m 26s
ubu-20 \| g++-10 \| c++20 \| libstdc++ \| ubuntu-20.04	5m 29s
ubu-24 \| clang++-18 \| c++20 \| libstdc++ \| ubuntu-24.04	6m 58s
ubu-24 \| clang++-18 \| c++23 \| libc++-18-dev \| ubuntu-24.04	15s
ubu-22 \| clang++-15 \| c++20 \| libstdc++ \| ubuntu-22.04	5m 29s
ubu-22 \| clang++-15 \| c++20 \| libc++-15-dev \| ubuntu-22.04	5m 39s
ubu-20 \| clang++-12 \| c++20 \| libstdc++ \| ubuntu-20.04	4m 51s
mac-14 \| clang++ \| c++2b \| default \| macos-14	12s
mac-13 \| clang++ \| c++2b \| default \| macos-13	5m 30s
mac-13 \| clang++-15 \| c++2b \| default \| macos-13	7m 4s
win-22 \| cl.exe \| c++latest \| default \| windows-2022	4m 27s
win-22 \| cl.exe \| c++20 \| default \| windows-2022	4m 27s
aggregate-results	6s
	1h 2m 1s

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