Fix PadUsing::next_back

[phimuemue]

This is a consolidated version of @Takashiidobe's #1079. Thanks, @Takashiidobe, for your effort.

• Test next, next_back explicitly: Simulate pad_usings result with a Vec and assert that they behave the same wrt next/next_back.

• Fix: Store elements_from_next and elements_from_next_back.

  The story is: total_consumed = elements_from_next + elements_from_next_back. If total_consumed >= elements_required, we can be sure that we only need to touch iter (and not filler). See Fix pad_using bug producing items when it shouldn't #1079 for more discussion.

  For simplicity, only implement next, next_back and size_hint for now.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.68%. Comparing base (6814180) to head (320edc9).
⚠️ Report is 173 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1082      +/-   ##
==========================================
- Coverage   94.38%   93.68%   -0.70%     
==========================================
  Files          48       50       +2     
  Lines        6665     6322     -343     
==========================================
- Hits         6291     5923     -368     
- Misses        374      399      +25     

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[phimuemue]

Hi @jswrenn, gentle nudge about this.

If it helps, you can consider this reviewed.

I did not/do not know how I could take @Takashiidobe's original PR and rebase it to this one. And afaik I cannot just push to this repo.

[jswrenn]

Thanks for the nudge, and sorry for the absence; I've been ramping up in a new day job. This looks fantastic! Can you just add Fixes: #1065 to the commit message?

[phimuemue]

Thanks for the nudge, and sorry for the absence; I've been ramping up in a new day job.

Nevermind. Congrats on the new job.

Can you just add Fixes: #1065 to the commit message?

Done.

Sidenote: Should we publish a new version soon? Would simplify CI (currently always complains about semver-checks).