Make Display format of Duration conform better to ISO 8601

[pitdicker]

The Display format of Duration currently converts all values greater than 24 hours to a value with days. But in ISO 8601 P1D has a different meaning from PT86400S.

If we get a duration type that properly supports the ISO 8601 format with different components (#1282), I think we should fix the simple Duration type to always write only seconds. We later may be able to share the parser and a FromStr implementation.

I ported the code to count the significant fractional digits in nanos from #1290.
It should find a shared place once one of these PRs has landed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (58f1f37) 93.41% compared to head (fab1030) 91.51%.
Report is 51 commits behind head on main.

❗ Current head fab1030 differs from pull request most recent head 0454b09. Consider uploading reports for the commit 0454b09 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1328      +/-   ##
==========================================
- Coverage   93.41%   91.51%   -1.91%     
==========================================
  Files          34       38       +4     
  Lines       16754    17316     +562     
==========================================
+ Hits        15651    15846     +195     
- Misses       1103     1470     +367     

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[djc]

What's the point of this loop?

[pitdicker]

To count the number of significant digits, and removing all trailing zero's. I.e. repeatedly divide nanos by 10.
I do not know a cleaner way to write it.