Benchmark `.nth[_back](n)` with inputs `n` #868

Job	Run time
msrv	37s
check	13s
check format	13s
check (--no-default-features)	13s
doc	13s
check (--no-default-features --features use_alloc)	16s
semver-checks	17s
check (--all-targets --all-features)	28s
test	49s
All checks succeeded	1s
	3m 20s

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