📝 Add design thoughts for `bitset` #1068

Job	Run time
valgrind	4m 23s
quality_checks_pass	59s
build_and_test_24 (clang, 18, 23, libstdc++, Debug)	6m 28s
mutate	5m 59s
build_and_test_24 (clang, 20, 23, libc++, Debug)	7m 55s
build_and_test_24 (clang, 18, 23, libc++, Debug)	7m 42s
build_and_test_24 (clang, 19, 23, libc++, Debug)	6m 23s
build_and_test_24 (clang, 19, 23, libstdc++, Debug)	5m 55s
build_and_test_24 (clang, 21, 23, libstdc++, Debug)	6m 13s
build_and_test_24 (clang, 22, 23, libstdc++, Debug)	7m 31s
build_and_test_24 (clang, 20, 23, libstdc++, Debug)	8m 47s
sanitize (address, gcc)	5m 0s
build_and_test_24 (gcc, 12, 23, libstdc++, Debug)	7m 38s
sanitize (thread, clang)	7m 24s
sanitize (thread, gcc)	7m 23s
sanitize (undefined, gcc)	5m 51s
build_and_test_24 (clang, 21, 23, libc++, Debug)	6m 42s
sanitize (address, clang)	6m 43s
sanitize (undefined, clang)	6m 50s
build_and_test_24 (clang, 22, 23, libc++, Debug)	6m 23s
build_and_test_24 (gcc, 14, 23, libstdc++, Debug)	5m 26s
build_and_test_24 (gcc, 13, 23, libstdc++, Debug)	4m 24s
merge_ok	1s
	2h 18m 0s

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