Bump actions/checkout from 3 to 4 #27

Job	Run time
rustfmt	22s
clippy	20s
Test 1.56 on ubuntu-latest ()	3m 11s
Test 1.56 on ubuntu-latest (--no-default-features)	2m 39s
Test stable on ubuntu-latest ()	29s
Test stable on ubuntu-latest (--no-default-features)	30s
Test nightly on ubuntu-latest ()	31s
Test nightly on ubuntu-latest (--no-default-features)	26s
Test 1.56 on macos-latest ()	3m 15s
Test 1.56 on macos-latest (--no-default-features)	3m 11s
Test stable on macos-latest ()	51s
Test stable on macos-latest (--no-default-features)	46s
Test nightly on macos-latest ()	2m 31s
Test nightly on macos-latest (--no-default-features)	46s
Test 1.56 on windows-latest ()	3m 30s
Test 1.56 on windows-latest (--no-default-features)	3m 19s
Test stable on windows-latest ()	59s
Test stable on windows-latest (--no-default-features)	1m 40s
Test nightly on windows-latest ()	1m 10s
Test nightly on windows-latest (--no-default-features)	1m 2s
	31m 28s

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