Specialize `Powerset::nth` #908

Job	Run time
doc	14s
check	15s
check format	12s
check (--no-default-features)	14s
semver-checks	18s
check (--no-default-features --features use_alloc)	12s
msrv	44s
check (--all-targets --all-features)	26s
test	57s
All checks succeeded	1s
	3m 33s

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