Fix Structset to handle uuid ([16]byte) correctly

[Fs02]

Fixes #252:

• UUID package like https://github.com/google/uuid use [16] byte as underlying type
• Fix isZero() to handle byte array as if it were on the same depth

[codecov]

Codecov Report

Merging #253 (8a81031) into master (30b6827) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #253   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           56        56           
  Lines         3972      3974    +2     
=========================================
+ Hits          3972      3974    +2     

Impacted Files	Coverage Δ
util.go	100.00% <100.00%> (ø)

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