Made Version.SortableString() comparable directly with RelaxedVersion.SortableString()

[cmaglie]

Previously comparing a sortable string from a Version and a RelaxedVersion may have an undefined outcome.
Now it's possible to compare the sortable strings directly.
This is a breaking change, if a previous Version.SortableString() has been stored on the DB, it must be regenerated.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (aa0ac68) to head (fe12533).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #25   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           12        12           
  Lines         1126      1126           
=========================================
  Hits          1126      1126           

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