Implement `Unique::fold` #874

Job	Run time
doc	6s
msrv	42s
check	8s
check format	8s
semver-checks	2m 20s
test	58s
check (--no-default-features)	10s
check (--no-default-features --features use_alloc)	24s
check (--all-targets --all-features)	24s
All checks succeeded	1s
	5m 21s

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