Switch CI from GitLab to GitHub Actions #12

Job	Run time
set-image	1s
checks	1m 25s
clippy	1m 10s
bench-rust-nightly (bit-vec)	17m 25s
miri (bit-vec)	51m 32s
bench-rust-nightly (bytes)	16m 25s
miri (bytes)	2m 34s
tests	1m 44s
bench-rust-nightly (generic-array)	16m 9s
miri (generic-array)	2m 27s
bench-rust-nightly (derive)	16m 8s
miri (arbitrary)	2m 29s
bench-rust-nightly (max-encoded-len)	14m 51s
build-linux-ubuntu-amd64	1m 6s
publish-dry-run	1m 17s
publish-crate	0s
	2h 26m 43s

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