Add verification of signed integers via Why3

[mborland]

Closes: #172
Closes: #112

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.6%. Comparing base (e0b94f2) to head (dee1fee).
⚠️ Report is 3 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff           @@
##           develop    #179   +/-   ##
=======================================
  Coverage     97.6%   97.6%           
=======================================
  Files          166     166           
  Lines        12670   12670           
  Branches      4660    4660           
=======================================
  Hits         12356   12356           
  Misses         247     247           
  Partials        67      67           

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