feat(`FilterOk`): implement `DoubleEndedIterator` #942

Job	Run time
check format	4s
doc	14s
check	30s
check (--no-default-features)	7s
msrv	23s
semver-checks	12s
check (--no-default-features --features use_alloc)	8s
test	53s
check (--all-targets --all-features)	19s
All checks succeeded	1s
	2m 51s

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