refactor: Use `AtomicUsize` instead of `AtomicU64` #365

Job	Run time
check_minimal_dependency_versions	49s
check_msrv	49s
check	53s
coverage	1m 7s
test (ubuntu-latest)	3m 19s
test (macos-latest)	1m 30s
test (windows-latest)	3m 41s
	12m 8s

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