Tests

Separate walking a file from file IO #1024

Job	Run time
build (macOS-latest, stable)	3m 43s
build (macOS-latest, nightly)	3m 31s
build (macOS-latest, 1.70)	3m 40s
build (ubuntu-latest, stable)	2m 18s
build (ubuntu-latest, nightly)	2m 2s
build (ubuntu-latest, 1.70)	2m 30s
build (windows-latest, stable)	4m 39s
build (windows-latest, nightly)	3m 1s
build (windows-latest, 1.70)	4m 26s
cargo-mutants	2h 17m 0s
	2h 46m 50s

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