Add ElementsMatch, ElementsMatchBy

[senago]

Solves #586

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.31%. Comparing base (70b763e) to head (2a222db).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #589      +/-   ##
==========================================
+ Coverage   94.27%   94.31%   +0.03%     
==========================================
  Files          18       18              
  Lines        3178     3200      +22     
==========================================
+ Hits         2996     3018      +22     
  Misses        170      170              
  Partials       12       12              

Flag	Coverage Δ
unittests	94.31% <100.00%> (+0.03%)	⬆️

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[senago]

I'm actually not sure If we should count the elements. So is done in testify package: ElementsMatch. That's the intent - to have the same familiar semantics. Though personally I don't find this counting really helpful.