slabtop: add `-o`/`--once` #234

Job	Run time
cargo fmt --all -- --check	8s
cargo check (ubuntu-latest)	14s
cargo clippy -- -D warnings (ubuntu-latest)	19s
cargo check (macOS-latest)	22s
cargo clippy -- -D warnings (macOS-latest)	29s
Code Coverage (ubuntu-latest, unix)	1m 55s
cargo test (ubuntu-latest)	24s
cargo check (windows-latest)	1m 21s
cargo clippy -- -D warnings (windows-latest)	1m 46s
Code Coverage (macos-latest, macos)	3m 30s
cargo test (macOS-latest)	1m 5s
Code Coverage (windows-latest, windows)	5m 7s
cargo test (windows-latest)	1m 25s
	18m 5s

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