slabtop: add `-o`/`--once` #230

Job	Run time
cargo test (ubuntu-latest)	35s
cargo test (macOS-latest)	36s
Code Coverage (ubuntu-latest, unix)	1m 51s
cargo clippy -- -D warnings (ubuntu-latest)	14s
Code Coverage (macos-latest, macos)	1m 38s
cargo test (windows-latest)	39s
cargo check (ubuntu-latest)	11s
Code Coverage (windows-latest, windows)	1m 49s
cargo clippy -- -D warnings (macOS-latest)	31s
cargo clippy -- -D warnings (windows-latest)	1m 29s
cargo check (macOS-latest)	22s
cargo fmt --all -- --check	8s
cargo check (windows-latest)	1m 21s
	11m 24s

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