Implement `FlattenOk::{fold, rfold}` #906

Job	Run time
msrv	42s
test	56s
doc	13s
check	13s
check (--no-default-features)	15s
semver-checks	2m 27s
check (--no-default-features --features use_alloc)	20s
check (--all-targets --all-features)	28s
check format	12s
All checks succeeded	1s
	5m 47s

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