Add constexpr to some str_view class member functions

[rmisev]

• add UPA_CONSTEXPR_14 macro
• add workflow to compile with g++ using C++14

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (28ee85f) 99.72% compared to head (459eafe) 99.72%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #23   +/-   ##
=======================================
  Coverage   99.72%   99.72%           
=======================================
  Files          15       15           
  Lines        2217     2217           
=======================================
  Hits         2211     2211           
  Misses          6        6           

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