Switch CI from GitLab to GitHub Actions #15

Job	Run time
set-image	1s
clippy	1m 9s
bench-rust-nightly (bit-vec)	17m 9s
checks	1m 28s
bench-rust-nightly (bytes)	16m 5s
miri (bit-vec)	49m 13s
bench-rust-nightly (generic-array)	16m 44s
tests	1m 50s
miri (bytes)	2m 47s
bench-rust-nightly (derive)	16m 15s
miri (generic-array)	2m 44s
bench-rust-nightly (max-encoded-len)	16m 5s
miri (arbitrary)	2m 30s
build-linux-ubuntu-amd64	1m 10s
publish-dry-run	1m 22s
	2h 26m 32s

Provide feedback