Resolve the C4800 warnings when compiled with MSVC compilers

[fktn-k]

This PR fixes the C4800 warnings when this library is compiled with a MSVC compiler as reported in the issue #429.
The warnings are now resolved by static_casting the result of std::isalnum() as bool.
Furthermore, the test suite is now built with the compile option /w44800 to avoid a possible regression in the future (the C4800 warning is desabled by default and even with the compile option /W4).

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• Changes are described in the pull request or in a referenced issue.
• The test suite compiles and runs without any error.
• The code coverage on your branch is 100%.
• The documentation is updated if you added/changed a feature.

Please don't

• The C++11 support varies between different compilers and versions. Please note the list of supported compilers. Some compilers like GCC 4.7 (and earlier), Clang 3.3 (and earlier), or Microsoft Visual Studio 13.0 and earlier are known not to work due to missing or incomplete C++11 support. Please refrain from proposing changes that work around these compiler's limitations with #ifdefs or other means.
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[github-actions]

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[fktn-k]

macos-12 is now deprecated and it's during the scheduled brownout period now.
actions/runner-images#10721
That's why all the workflow jobs using the macos-12 runner are failing...
I'll re-run those failed jobs tomorrow.