feat(`FilterMapOk`): implement `DoubleEndedIterator` #944

Job	Run time
check format	4s
check	7s
doc	10s
check (--no-default-features)	6s
check (--no-default-features --features use_alloc)	7s
msrv	25s
semver-checks	2m 22s
check (--all-targets --all-features)	20s
test	53s
All checks succeeded	1s
	4m 35s

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