Implement `Combinations::nth` #864

Job	Run time
check format	15s
doc	16s
check	16s
check (--no-default-features)	15s
msrv	32s
check (--no-default-features --features use_alloc)	15s
semver-checks	17s
check (--all-targets --all-features)	27s
test	55s
All checks succeeded	1s
	3m 29s

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