Disallow duplicated enum variant indices #251

Job	Run time
set-image	1s
clippy	1m 4s
fmt	55s
checks	1m 11s
bench-rust-nightly (bit-vec)	3m 8s
bench-rust-nightly (bytes)	3m 3s
miri (bit-vec)	3m 5s
bench-rust-nightly (generic-array)	3m 1s
tests	1m 19s
miri (bytes)	2m 40s
bench-rust-nightly (derive)	3m 0s
miri (generic-array)	2m 54s
bench-rust-nightly (max-encoded-len)	3m 2s
miri (arbitrary)	2m 45s
build-linux-ubuntu-amd64	0s
publish-dry-run	0s
	31m 8s

Provide feedback