feat(`FilterMapOk`): implement `DoubleEndedIterator` #945

Job	Run time
doc	15s
check	14s
test	53s
check format	7s
check (--no-default-features)	7s
check (--no-default-features --features use_alloc)	6s
check (--all-targets --all-features)	20s
semver-checks	10s
msrv	26s
All checks succeeded	1s
	2m 39s

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