CI: Add GitHub Actions workflow #2

Job	Run time
rustfmt	16s
rustdoc (stable)	46s
rustdoc (beta)	1m 10s
test (ubuntu-latest, stable)	1m 30s
rustdoc (nightly)	1m 7s
clippy (ubuntu-latest, stable)	43s
test (ubuntu-latest, beta)	59s
clippy (ubuntu-latest, beta)	48s
test (ubuntu-latest, nightly)	1m 2s
clippy (ubuntu-latest, nightly)	49s
test (windows-latest, stable)	2m 23s
clippy (windows-latest, stable)	1m 45s
clippy (windows-latest, beta)	1m 44s
test (windows-latest, beta)	2m 26s
clippy (windows-latest, nightly)	1m 26s
test (windows-latest, nightly)	2m 14s
clippy (macos-latest, stable)	1m 49s
test (macos-latest, stable)	1m 26s
test (macos-latest, beta)	1m 3s
clippy (macos-latest, beta)	1m 7s
test (macos-latest, nightly)	1m 58s
clippy (macos-latest, nightly)	3m 13s
	31m 44s

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