In this PR, some refactorings have been made for better performance, including:

• cleaning up basic_node's destructor
• optimizing the process of scanning multiline scalars
• improving the scanning of scalar contents for type detection

Also, the Make target for benchmarking contains a wrong fixed path and performance of debug builds were not correctly measured.
However, the performance of debug builds seems to need some more work for trustworthy results, which is why the performance data of debug builds have been removed (possibly revoked in the future).

Pull Request Checklist

Read the CONTRIBUTING.md file for detailed information.

• 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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