Expand continuous integration

[mity]

The plan is:

• Add job to make Release build on Linux (gcc gives some warnings only on Release builds, e.g. about use of uninitialized variables)
• Change Windows builds to use nmake rather than Visual Studio solutions.
• One of the Windows builds should build Release and other one Debug to improve error/warning coverage by MSVC.
• Run our tests also on Windows builds.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (30945d8) 91.76% compared to head (e7c4392) 91.76%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #237   +/-   ##
=======================================
  Coverage   91.76%   91.76%           
=======================================
  Files           5        5           
  Lines        3402     3402           
=======================================
  Hits         3122     3122           
  Misses        280      280           

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