AtomicCell: Use atomic-maybe-uninit #3086

Job	Run time
features (msrv)	43s
codegen	3s
features (nightly)	26s
clippy	19s
careful	4m 48s
dependencies	16s
docs	11s
loom	50s
miri (channel)	25m 35s
rustfmt	3s
miri (others)	14m 6s
shellcheck	8s
san	12m 45s
test (1.63, ubuntu-latest)	10m 57s
test (1.63, windows-latest)	15m 10s
test (stable, ubuntu-latest)	9m 17s
test (stable, windows-latest)	13m 49s
test (nightly, ubuntu-latest)	8m 44s
test (nightly, windows-latest)	13m 22s
test (nightly, ubuntu-latest, i686-unknown-linux-gnu)	9m 51s
test (nightly, ubuntu-latest, armv7-unknown-linux-gnueabihf)	14m 54s
test (nightly, ubuntu-latest, armv5te-unknown-linux-gnueabi)	16m 18s
	2h 52m 35s

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