Implement `FlattenOk::{fold, rfold}` #893

Job	Run time
msrv	40s
doc	14s
check	6s
check format	5s
check (--no-default-features)	9s
check (--no-default-features --features use_alloc)	8s
semver-checks	10s
check (--all-targets --all-features)	19s
test	55s
All checks succeeded	1s
	2m 47s

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