zip_eq: add DoubleEndedIterator implementation

[petrosagg]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 94.07%. Comparing base (6814180) to head (6a24283).
⚠️ Report is 156 commits behind head on master.

Files with missing lines	Patch %	Lines
src/zip_eq_impl.rs	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1055      +/-   ##
==========================================
- Coverage   94.38%   94.07%   -0.32%     
==========================================
  Files          48       50       +2     
  Lines        6665     6260     -405     
==========================================
- Hits         6291     5889     -402     
+ Misses        374      371       -3     

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[petrosagg]

I just noticed the discussion in #531, closing since there are some good points raised there