add TagSet and Tag.Fill and an example

[muir]

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[codecov-commenter]

Codecov Report

Merging #3 (f9d5468) into main (7f2142f) will increase coverage by 0.95%.
The diff coverage is 78.57%.

@@            Coverage Diff             @@
##             main       #3      +/-   ##
==========================================
+ Coverage   74.04%   75.00%   +0.95%     
==========================================
  Files           3        3              
  Lines         131      196      +65     
==========================================
+ Hits           97      147      +50     
- Misses         21       31      +10     
- Partials       13       18       +5     

Impacted Files	Coverage Δ
parsetag.go	79.26% <78.57%> (-8.97%)	⬇️

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