cmp.rs: simplify by .ok_or #788

Job	Run time
cargo clippy -- -D warnings (macOS-latest)	14s
cargo check (ubuntu-latest)	11s
cargo test (ubuntu-latest)	1m 15s
cargo clippy -- -D warnings (windows-latest)	1m 9s
cargo clippy -- -D warnings (ubuntu-latest)	18s
cargo fmt --all -- --check	4s
Code Coverage (windows-latest, windows)	6m 30s
cargo check (macOS-latest)	15s
cargo test (macOS-latest)	3m 53s
Code Coverage (macos-latest, macos)	2m 36s
cargo check (windows-latest)	38s
Code Coverage (ubuntu-latest, unix)	1m 57s
cargo test (windows-latest)	4m 33s
	23m 33s

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