Disallow duplicated enum variant indices #252

Job	Run time
set-image	1s
checks	1m 11s
clippy	1m 3s
bench-rust-nightly (bit-vec)	16m 58s
fmt	56s
bench-rust-nightly (bytes)	16m 3s
tests	1m 59s
bench-rust-nightly (generic-array)	16m 10s
miri (bit-vec)	5h 43m 20s
bench-rust-nightly (derive)	15m 57s
miri (bytes)	2m 48s
bench-rust-nightly (max-encoded-len)	16m 16s
miri (generic-array)	2m 54s
miri (arbitrary)	2m 38s
build-linux-ubuntu-amd64	1m 6s
publish-dry-run	1m 2s
	7h 20m 22s

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