Accept bare labels as tags with an empty value

[mtfishman]

Summary

Adds bare labels to the accepted tag inputs, so tags = "i" is shorthand for tags = "i" => "": a String or Symbol, a collection of them, or a collection mixing them with key => value pairs, each becomes a tag with an empty value. settags accepts the same forms, and an empty-value tag now prints as just its key (flux rather than flux=>).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.29%. Comparing base (f748ab0) to head (dc88040).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #226      +/-   ##
==========================================
+ Coverage   77.21%   77.29%   +0.07%     
==========================================
  Files          29       29              
  Lines        1712     1718       +6     
==========================================
+ Hits         1322     1328       +6     
  Misses        390      390              

Flag	Coverage Δ
docs	27.11% <0.00%> (-0.10%)	⬇️

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