Expand continuous integration

[mity]

The plan is:

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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