Separate scalar parsing from lexical analysis

[fktn-k]

In this PR, scalar parsing has been separated from lexical analysis, and a lot of unnecessary conditional branches and heap allocations are thus reduced since the separation reduces complexities in each operations. The performance of the whole deserialization process has therefore been increased by 5 Mi/s on average.	parsed YAML file	Before this PR	After this PR
ubuntu.yml	50.4621Mi/s	55.1393Mi/s
citm_catalog.json	75.5534Mi/s	82.9931Mi/s
citm_catalog.yml	32.7921Mi/s	35.152Mi/s

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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.
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[github-actions[bot]]

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