Add (Un)Marshal functions to stats.TagSet

[mstoykov]

Also, sort the keys before marshaling them with SystemTagSet for
consistent output.

fixes #1602

[codecov-commenter]

Codecov Report

Merging #1604 into master will decrease coverage by 0.02%.
The diff coverage is 92.85%.

@@            Coverage Diff             @@
##           master    #1604      +/-   ##
==========================================
- Coverage   74.87%   74.85%   -0.03%     
==========================================
  Files         164      164              
  Lines       14035    14062      +27     
==========================================
+ Hits        10509    10526      +17     
- Misses       2994     3003       +9     
- Partials      532      533       +1     

Impacted Files	Coverage Δ
stats/system_tag.go	85.00% <92.85%> (+3.86%)	⬆️
lib/testutils/minirunner/minirunner.go	78.57% <0.00%> (-3.58%)	⬇️
lib/executor/vu_handle.go	93.69% <0.00%> (-1.81%)	⬇️
js/runner.go	83.04% <0.00%> (-0.70%)	⬇️
stats/cloud/collector.go	78.74% <0.00%> (-0.55%)	⬇️

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[na--]

LGTM