feat(FilterMapOk): implement DoubleEndedIterator

[Xenira]

DoubleEndedIterator implementation for FilterMapOk analog to #948.

Hope rev() is the right thing here, as rfind_map does not exist (#949).

Refs: #947

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.52%. Comparing base (6814180) to head (aa547c6).
Report is 98 commits behind head on master.

Additional details and impacted files

@@ Coverage Diff @@## master #950 +/- ##
==========================================
+ Coverage 94.38% 94.52% +0.13% 
==========================================
Files 48 49 +1 Lines 6665 7046 +381 ==========================================
+ Hits 6291 6660 +369 - Misses 374 386 +12 

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[Philippe-Cholet]

You did not need to make a merge commit.

.by_ref().rev().find_map( is verbose but it's not unprecedented like for Unique::next_back and it's fine.

[Xenira]

You did not need to make a merge commit.

Wanted to rebase, but messed up. Like my pr's to be up to date :)

[Philippe-Cholet]

No prob, the commit history is not that clean anyway.

[scottmcm]

Hmm, maybe the other option would be .filter_map(p).next_back(), inspired by rust-lang/rust#49098 ?

The rev is probably fine, though, so probably doesn't need to be changed.

[Xenira]

Benchmarked against the filter_map variant and its mostly in noise threshold. (2 cases 2% faster but I guess, thats because different base load on my system).

The rev() is more readable though.