AtomicCell: Use atomic-maybe-uninit #3113

Job	Run time
codegen	3s
clippy	13s
careful	5m 19s
dependencies	17s
docs	12s
features (msrv)	49s
loom	49s
features (nightly)	23s
miri (channel)	23m 7s
rustfmt	10s
miri (others)	14m 31s
san	14m 2s
shellcheck	7s
test (1.63, ubuntu-latest)	11m 39s
test (1.63, windows-latest)	15m 20s
test (stable, ubuntu-latest)	10m 18s
test (stable, windows-latest)	14m 22s
test (nightly, ubuntu-latest)	9m 34s
test (nightly, windows-latest)	13m 31s
test (nightly, ubuntu-latest, i686-unknown-linux-gnu)	10m 43s
test (nightly, ubuntu-latest, armv7-unknown-linux-gnueabihf)	15m 37s
test (nightly, ubuntu-latest, armv5te-unknown-linux-gnueabi)	17m 12s
test (stable, ubuntu-latest, sparc64-unknown-linux-gnu)	14m 29s
	3h 12m 47s

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