Implement `Combinations::nth` #859

Job	Run time
check format	12s
check	15s
msrv	38s
check (--no-default-features)	12s
doc	13s
test	53s
check (--no-default-features --features use_alloc)	12s
check (--all-targets --all-features)	25s
semver-checks	15s
All checks succeeded	1s
	3m 16s

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