Disallow duplicated enum variant indices #205

Job	Run time
set-image	1s
fmt	1m 37s
clippy	1m 39s
checks	1m 49s
bench-rust-nightly (bit-vec)	17m 32s
miri (bit-vec)	5h 38m 5s
bench-rust-nightly (bytes)	16m 57s
tests	1m 31s
miri (bytes)	3m 6s
bench-rust-nightly (generic-array)	16m 54s
miri (generic-array)	3m 26s
bench-rust-nightly (derive)	16m 51s
bench-rust-nightly (max-encoded-len)	16m 55s
miri (arbitrary)	3m 39s
build-linux-ubuntu-amd64	0s
publish-dry-run	0s
	7h 20m 2s

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