Boost coverage for struct CowStr #15

Job	Run time
Preflight crate publish (ubuntu-latest, stable)	15s
Unit tests (windows-latest, stable)	53s
Unit tests (windows-latest, 1.72.0)	43s
Preflight docs.rs build	10s
Clippy	13s
Unit tests (macos-latest, stable)	35s
Unit tests (macos-latest, 1.72.0)	34s
Unit tests with minimum versions of direct dependencies (windows-latest)	35s
Unit tests (ubuntu-latest, stable)	23s
Unit tests with minimum versions of direct dependencies (macos-latest)	28s
Enforce Rust code format	11s
Unit tests (ubuntu-latest, 1.72.0)	23s
License / vulnerability audit (advisories)	20s
Unit tests with minimum versions of direct dependencies (ubuntu-latest)	17s
License / vulnerability audit (bans licenses sources)	17s
Check for unused dependencies	13s
	6m 30s

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