AtomicCell: Use atomic-maybe-uninit #3106

Job	Run time
features (msrv)	51s
features (nightly)	24s
codegen	2s
clippy	12s
careful	5m 10s
dependencies	16s
docs	11s
loom	47s
miri (channel)	22m 52s
rustfmt	4s
miri (others)	14m 21s
san	14m 1s
test (1.63, ubuntu-latest)	11m 38s
test (1.63, windows-latest)	15m 9s
shellcheck	6s
test (stable, ubuntu-latest)	10m 4s
test (stable, windows-latest)	13m 39s
test (nightly, ubuntu-latest)	9m 33s
test (nightly, windows-latest)	13m 20s
test (nightly, ubuntu-latest, i686-unknown-linux-gnu)	10m 35s
test (nightly, ubuntu-latest, armv7-unknown-linux-gnueabihf)	15m 35s
test (nightly, ubuntu-latest, armv5te-unknown-linux-gnueabi)	16m 37s
test (stable, ubuntu-latest, sparc64-unknown-linux-gnu)	14m 41s
	3h 10m 8s

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