Simplify `Permutations` #567

Job	Run time
test	46s
check format	11s
msrv	1m 59s
check	6s
check (--no-default-features)	21s
check (--no-default-features --features use_alloc)	17s
check (--all-targets --all-features)	37s
All checks succeeded	2s
	4m 19s

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