test: add more unit tests

[rafamizes]

Closes #35

[codecov]

Codecov Report

Merging #36 (56d8eaa) into master (91c583a) will increase coverage by 1.12%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #36      +/-   ##
==========================================
+ Coverage   89.52%   90.65%   +1.12%     
==========================================
  Files          62       62              
  Lines         974      974              
==========================================
+ Hits          872      883      +11     
+ Misses        102       91      -11     

Impacted Files	Coverage Δ
lib/src/bit/add.dart	100.00% <0.00%> (+11.11%)	⬆️
lib/byte.dart	100.00% <0.00%> (+18.18%)	⬆️
lib/src/bit/nshl.dart	100.00% <0.00%> (+25.00%)	⬆️
lib/src/bit/nshr.dart	100.00% <0.00%> (+25.00%)	⬆️
lib/src/bit/nshrs.dart	100.00% <0.00%> (+25.00%)	⬆️
lib/src/bit/sub.dart	100.00% <0.00%> (+55.55%)	⬆️

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