slabtop: add `-o`/`--once` #302

Job	Run time
cargo fmt --all -- --check	19s
cargo fmt --all -- --check	19s
cargo clippy -- -D warnings (ubuntu-latest)	27s
cargo clippy -- -D warnings (ubuntu-latest)	27s
cargo check (ubuntu-latest)	19s
cargo check (ubuntu-latest)	19s
cargo clippy -- -D warnings (macOS-latest)	25s
cargo clippy -- -D warnings (macOS-latest)	25s
Code Coverage (ubuntu-latest, unix)	36s
Code Coverage (ubuntu-latest, unix)	2m 2s
cargo check (macOS-latest)	21s
cargo check (macOS-latest)	21s
cargo test (ubuntu-latest)	33s
cargo test (ubuntu-latest)	33s
cargo clippy -- -D warnings (windows-latest)	1m 29s
cargo clippy -- -D warnings (windows-latest)	1m 29s
Code Coverage (macos-latest, macos)	45s
Code Coverage (macos-latest, macos)	1m 42s
cargo test (macOS-latest)	29s
cargo test (macOS-latest)	29s
cargo check (windows-latest)	1m 38s
cargo check (windows-latest)	1m 38s
Code Coverage (windows-latest, windows)	40s
Code Coverage (windows-latest, windows)	4m 40s
cargo test (windows-latest)	1m 36s
cargo test (windows-latest)	1m 36s
	25m 37s

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