Further improvements of input handlings

[fktn-k]

This PR has improved input handling, as additional works which are untouched in #296.
The following changes have been made:

• input_handler::test_next_char() function has been replaced with input_handler::peek_next() function to more generally access the next input character without moving the current position.
• Preparation of buffers to be filled with input characters have been improved by reducing the number of calls during the initialization process of the lexer for optimization purposes.
• String manipulation in the lexer has been optimized.

Note that the public APIs are not affected by the above changes.

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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.

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[github-actions]

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[coveralls]

Pull Request Test Coverage Report for Build 8492707297

Details

• 287 of 287 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 8477959635:	0.0%
Covered Lines:	3046
Relevant Lines:	3046

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