#237 Support char8_t in deserialization

[fktn-k]

As described in #237, since C++20, the string literal u8"" has changed to return a char8_t[N] object, instead of a char[N] object, which has broken backward compatiblity with the code compiled with C++17 or earlier.
fkYAML is intended to work C++11 and later, and so this PR has added a workaround to resolve the issue described above.

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• [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%.
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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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• Please do not open pull requests that address multiple issues.

[coveralls]

coverage: 100.0%. remained the same when pulling 17274688f7cef5defb9d6522b0bdbff2309e7058 on feature/237_support_char8_t_in_deserialization into efe56b488818e529b14388354aabd954084abc9f on develop.