`Powerset::fold` specialization #457

Job	Run time
check format	13s
msrv	1m 27s
check	8s
test	1m 11s
check (--no-default-features)	16s
check (--no-default-features --features use_alloc)	6s
check (--all-targets --all-features)	22s
bors build finished	1s
	3m 44s

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