Addition: dectests + implementation

[barucden]

Requires #75

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.66%. Comparing base (73e7c86) to head (ecdb301).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #80      +/-   ##
==========================================
+ Coverage   99.64%   99.66%   +0.02%     
==========================================
  Files          11       11              
  Lines         280      301      +21     
==========================================
+ Hits          279      300      +21     
  Misses          1        1              

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[barucden]

div(::BigInt, ::BigInt, RoundFromZero) was added in Julia 1.9. How should I deal with that method missing in Julia 1.8? I wouldn't want to remove those tests altogether.