Make `Permutations` lazy #572

Job	Run time
test	42s
msrv	1m 34s
check format	12s
check	9s
check (--no-default-features)	8s
check (--no-default-features --features use_alloc)	10s
check (--all-targets --all-features)	18s
All checks succeeded	1s
	3m 14s

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