Add `@hashable` metafunction #692

Job	Run time
ubu-24 \| g++-14 \| c++2b \| libstdc++ \| ubuntu-24.04	11m 5s
ubu-24 \| g++-13 \| c++2b \| libstdc++ \| ubuntu-24.04	6m 15s
ubu-20 \| g++-10 \| c++20 \| libstdc++ \| ubuntu-20.04	4m 58s
ubu-24 \| clang++-18 \| c++20 \| libstdc++ \| ubuntu-24.04	7m 11s
ubu-24 \| clang++-18 \| c++23 \| libc++-18-dev \| ubuntu-24.04	7m 11s
ubu-22 \| clang++-15 \| c++20 \| libstdc++ \| ubuntu-22.04	5m 23s
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 49s
mac-14 \| clang++ \| c++2b \| default \| macos-14	2m 43s
mac-13 \| clang++ \| c++2b \| default \| macos-13	6m 15s
mac-13 \| clang++-15 \| c++2b \| default \| macos-13	11m 18s
win-22 \| cl.exe \| c++latest \| default \| windows-2022	4m 32s
win-22 \| cl.exe \| c++20 \| default \| windows-2022	4m 25s
aggregate-results	3s
	1h 21m 47s

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