Checks for zeroness

[projekter]

Fix #331
I also added equality definitions - if this is desired. However, it would be a breaking change - iszero and isone were simply not defined, so this is fine, but comparison with Zero went to the === fallback before and returned false. So I understand if you'd rather not have these comparisons.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.36%. Comparing base (cc1be0e) to head (881c5dc).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #332   +/-   ##
=======================================
  Coverage   90.35%   90.36%           
=======================================
  Files          22       22           
  Lines        2251     2253    +2     
=======================================
+ Hits         2034     2036    +2     
  Misses        217      217           

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[blegat]

Failure on nightly shouldn't be related