Improve conversion from scalars to native types

[fktn-k]

In this PR, conversions from scalars to native data types (int or bool) have been reimplemented, which happens when each scalar token is found in an input buffer during the deserialization process.
The conversions required unnecessary std::string instance creations before, which causes a number of heap allocations even when the instanciations is just the same as duplication of some part of an input buffer.
Most of the unnecessary heap allocations have therefore been replaced with references using iterators with no changes in public APIs and behaviors from the viewpoint of library users.

One exception is that conversions from a scalar to a floating point value still requires unnecessary heap allocations by default.
If std::from_chars() (since C++17) is available, the library now call it and thus no heap allocation happens.
In the other cases, however, the library calls std::stof() or std::stod() depending on the destination type which require a std::string object for a null-terminated string.
Although using some third party libraries like fast_float or implementing a similar feature inside the library can resolve this issue, that either consts a lot or leads to other issues (licensing, dependency management, etc...).
So, I put aside the issue and moved the latest implementations (with some sophistication though).

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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%.
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• 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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