✨ Add `into_variant` #375

Job	Run time
quality_checks_pass	58s
build_and_test_22 (clang, 14, 20, libc++, Debug)	3m 16s
build_and_test_24 (clang, 16, 20, libstdc++, Debug)	3m 39s
build_and_test_22 (clang, 15, 20, libstdc++, Debug)	2m 51s
sanitize (undefined, clang)	3m 40s
build_and_test_24 (clang, 16, 20, libc++, Debug)	3m 48s
valgrind	7m 48s
sanitize (undefined, gcc)	4m 14s
build_and_test_22 (clang, 15, 20, libc++, Debug)	3m 46s
build_and_test_24 (clang, 17, 20, libstdc++, Debug)	3m 36s
sanitize (address, clang)	3m 36s
build_and_test_24 (clang, 17, 20, libc++, Debug)	4m 4s
sanitize (address, gcc)	3m 59s
build_and_test_24 (clang, 18, 20, libstdc++, Debug)	3m 31s
sanitize (thread, clang)	3m 57s
build_and_test_24 (clang, 18, 20, libc++, Debug)	4m 10s
sanitize (thread, gcc)	4m 19s
build_and_test_24 (gcc, 12, 20, libstdc++, Debug)	3m 36s
build_and_test_24 (gcc, 13, 20, libstdc++, Debug)	3m 50s
merge_ok	6s
	1h 12m 44s

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