Use variable templates instead of structs for traits

[ZXShady]

Variable templates are simpler for the compiler and just better to read and write.

Note i did not change is_range to variable template since it is part of public API therefore this is an API break.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.98%. Comparing base (dde8220) to head (484c359).
Report is 1 commits behind head on devel.

Additional details and impacted files

@@           Coverage Diff           @@
##            devel    #2975   +/-   ##
=======================================
  Coverage   90.98%   90.98%           
=======================================
  Files         198      198           
  Lines        8599     8599           
=======================================
  Hits         7823     7823           
  Misses        776      776           

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[horenmar]

The changes look fine, but the new variables should follow the stdlib convention of using _v suffix for constexpr bool variables.