Add `Weekday::days_since`.

[pitdicker]

trafficstars

This resurrects the work in https://github.com/chronotope/chrono/pull/216 by @clarfonthey.

WeekDay::days_since doesn't offer any new functionality, but I quite like its API.

Fixes https://github.com/chronotope/chrono/issues/201.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.80%. Comparing base (0cfc405) to head (ba7ec9d).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1249   +/-   ##
=======================================
  Coverage   91.80%   91.80%           
=======================================
  Files          37       37           
  Lines       18148    18151    +3     
=======================================
+ Hits        16661    16664    +3     
  Misses       1487     1487           

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[pitdicker]

Hmm, it must be late :laughing:. I'll fix it to actually work, and merge it with the not-public WeekDay::num_days_from that does the same thing.

[pitdicker]

'Co-authored-by:` is new for me, I'll have to learn about that.

[djc]

https://docs.github.com/en/pull-requests/committing-changes-to-your-project/creating-and-editing-commits/creating-a-commit-with-multiple-authors

[pitdicker]

Thank you! It doesn't render special in Git Gui, but I guess this is it...