Switch CI from GitLab to GitHub Actions #9

Job	Run time
set-image	1s
checks	1m 30s
clippy	1m 35s
miri (bit-vec)	52m 8s
bench-rust-nightly (bit-vec)	18m 0s
tests	2m 57s
miri (bytes)	2m 52s
bench-rust-nightly (bytes)	16m 55s
miri (generic-array)	2m 55s
bench-rust-nightly (generic-array)	16m 54s
miri (arbitrary)	2m 52s
bench-rust-nightly (derive)	17m 14s
bench-rust-nightly (max-encoded-len)	17m 17s
build-linux-ubuntu-amd64	1m 22s
publish-dry-run	1m 20s
publish-crate	0s
	2h 35m 52s

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