cmp: print verbose diffs as we find them #322

Job	Run time
cargo fmt --all -- --check	4s
cargo check (ubuntu-latest)	6s
cargo clippy -- -D warnings (ubuntu-latest)	8s
Code Coverage (ubuntu-latest, unix)	1m 58s
cargo check (macOS-latest)	10s
cargo clippy -- -D warnings (macOS-latest)	11s
Code Coverage (macos-latest, macos)	2m 22s
GNU test suite	26s
cargo check (windows-latest)	32s
cargo clippy -- -D warnings (windows-latest)	44s
Code Coverage (windows-latest, windows)	5m 42s
cargo test (ubuntu-latest)	33s
cargo test (macOS-latest)	2m 58s
cargo test (windows-latest)	2m 55s
	18m 49s

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