tidy variant checks

[tomjnixon]

closes #191

Perhaps this should be called isVariantType instead? It's nice if the name of this matches std::holds_alternative which will eventually replace this.

Code size changes are not dramatic, but every little helps.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (3f94a9b) 89.91% compared to head (aaf0c8c) 89.91%.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #195   +/-   ##
=======================================
  Coverage   89.91%   89.91%           
=======================================
  Files         125      125           
  Lines        5812     5812           
=======================================
  Hits         5226     5226           
  Misses        586      586           

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