slabtop: add `-o`/`--once` #231

Job	Run time
cargo clippy -- -D warnings (ubuntu-latest)	14s
cargo check (ubuntu-latest)	12s
cargo fmt --all -- --check	5s
cargo clippy -- -D warnings (macOS-latest)	1m 0s
Code Coverage (ubuntu-latest, unix)	1m 51s
cargo check (macOS-latest)	19s
Code Coverage (macos-latest, macos)	1m 27s
cargo clippy -- -D warnings (windows-latest)	1m 43s
cargo check (windows-latest)	1m 13s
cargo test (ubuntu-latest)	26s
Code Coverage (windows-latest, windows)	1m 36s
cargo test (macOS-latest)	1m 6s
cargo test (windows-latest)	1m 14s
	12m 26s

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