This PR has fixed parse errors on flow container keys like the following valid YAML snippet:

{foo: bar}: 123
[true,false]: 3.14

The root cause is that the fkYAML deserializer wrongly assumed that flow containers weren't used as a mapping key.
To validate the fix, the test suite has also been updated by adding several relevant test cases for the deserializer.

Pull Request Checklist

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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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