Disallow duplicated enum variant indices #250

Job	Run time
set-image	1s
fmt	55s
clippy	1m 0s
checks	1m 0s
bench-rust-nightly (bit-vec)	1m 1s
bench-rust-nightly (bytes)	1m 2s
tests	59s
bench-rust-nightly (generic-array)	1m 0s
miri (bit-vec)	1m 0s
bench-rust-nightly (derive)	1m 2s
miri (bytes)	1m 0s
bench-rust-nightly (max-encoded-len)	1m 0s
miri (generic-array)	1m 0s
miri (arbitrary)	57s
build-linux-ubuntu-amd64	1s
publish-dry-run	1s
	12m 59s

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