find: Implement `-[no]user` and `-[no]group` predicates. #935

Job	Run time
Code coverage (ubuntu-latest, nightly, --all-features)	2m 7s
cargo test (ubuntu-latest)	38s
cargo check (ubuntu-latest)	12s
cargo test (macOS-latest)	32s
cargo fmt --all -- --check	3s
cargo check (macOS-latest)	16s
cargo test (windows-latest)	33s
cargo check (windows-latest)	1m 38s
cargo clippy -- -D warnings	17s
	6m 16s

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