AtomicCell: Use atomic-maybe-uninit #3100

Job	Run time
codegen	3s
test (1.63, ubuntu-latest)	11m 32s
test (1.63, windows-latest)	15m 17s
clippy	14s
test (stable, ubuntu-latest)	10m 15s
dependencies	15s
features (msrv)	46s
test (stable, windows-latest)	13m 39s
features (nightly)	24s
miri (channel)	22m 28s
test (nightly, ubuntu-latest)	9m 39s
rustfmt	5s
test (nightly, windows-latest)	13m 37s
miri (others)	14m 40s
test (nightly, ubuntu-latest, i686-unknown-linux-gnu)	10m 37s
test (nightly, ubuntu-latest, armv7-unknown-linux-gnueabihf)	15m 14s
careful	5m 10s
test (nightly, ubuntu-latest, armv5te-unknown-linux-gnueabi)	17m 0s
docs	13s
test (stable, ubuntu-latest, sparc64-unknown-linux-gnu)	14m 23s
loom	53s
san	13m 55s
shellcheck	20s
	3h 10m 39s

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