various improvements to `__variant` #2647

Job	Run time
CPU (Windows) (msvc, Debug)	11m 38s
macos-CPU (MacOS) (clang, Release)	3m 42s
CPU (gcc 11, Debug)	4m 49s
CPU (gcc 11, Release)	4m 51s
CPU (clang 16, Release, ASAN)	6m 33s
CPU (Windows) (msvc, Release)	12m 22s
CPU (clang 16, Release)	5m 51s
CPU (gcc 13, Debug)	5m 9s
macos-CPU (MacOS) (clang, Debug)	3m 57s
CPU (gcc 11, Release, ASAN)	6m 18s
CPU (gcc 12, Release, TSAN)	5m 24s
CPU (clang 16, Debug)	5m 30s
CI (CPU) (MacOS)	4s
CI (CPU)	2s
CI (CPU) (Windows)	7s
	1h 16m 17s

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