slabtop: add `-o`/`--once` #330

Job	Run time
cargo check (ubuntu-latest)	13s
cargo test (ubuntu-latest)	24s
cargo check (macOS-latest)	19s
cargo clippy -- -D warnings (ubuntu-latest)	15s
cargo test (macOS-latest)	27s
cargo check (windows-latest)	1m 23s
cargo clippy -- -D warnings (macOS-latest)	17s
Code Coverage (ubuntu-latest, unix)	1m 59s
cargo test (windows-latest)	1m 32s
cargo clippy -- -D warnings (windows-latest)	1m 24s
Code Coverage (macos-latest, macos)	1m 43s
cargo fmt --all -- --check	13s
Code Coverage (windows-latest, windows)	4m 52s
	15m 1s

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