Bump actions/checkout from 3 to 4 #325

Job	Run time
Clippy	2m 36s
Check	44s
Rustfmt	8s
MSRV	4m 14s
spellcheck	13s
Test Suite (ubuntu-latest, stable)	8m 27s
Test Suite (ubuntu-latest, nightly)	9m 48s
Test Suite (macos-latest, stable)	11m 38s
Test Suite (macos-latest, nightly)	10m 41s
Test Suite (windows-latest, stable)	17m 26s
Test Suite (windows-latest, nightly)	14m 27s
	1h 20m 22s

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