Use likely/unlikely if available

[fktn-k]

This PR intoduces FK_YAML_LIKELY and FK_YAML_UNLIKELY macros which conditionally expand to:

• [[likely]] and [[unlikely]] C++ attributes if available (preferred since they should be compiler agnostic as part of C++ standard attributes)
• __builtin_expect(!!(expr), 1) and __builtin_expect(!!(expr), 0) invocations if __builtin_expect is available (like GCC like compilers)
• just !!(expr) if the above two aren't available (like MSVC compilers with C++11-17)

Along with the above change, some preprocessor macros have also been tidied.

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Pull Request Checklist

Read the CONTRIBUTING.md file for detailed information.

• [x] Changes are described in the pull request or in a referenced issue.
• [x] The test suite compiles and runs without any error.
• [x] The code coverage on your branch is 100%.
• [x] 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.
• Please refrain from proposing changes that would break YAML specifications. If you propose a conformant extension of YAML to be supported by the library, please motivate this extension.
• Please do not open pull requests that address multiple issues.

[github-actions[bot]]

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