Addresses issue #4163.

Fixes the deprecation warning for char_traits with signed and unsigned char. These types have been explicitly marked as deprecated in the latest version of LLVM.

More information on the deprecation can be found in this announcement from LLVM.

This creates a custom char_traits struct that will extend the std::char_traits struct. The primary template will simply use the std::char_traits for a given type, but the specializations will provide char_traits methods and traits for non-standard char types (unsigned and signed char).

Functions that relied on std::char_traits for these unsupported types have been updated to use the custom char_traits implementation.

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• Changes are described in the pull request, or an existing issue is referenced.
• The test suite compiles and runs without error.
• Code coverage is 100%. Test cases can be added by editing the test suite.
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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.
• Specifically, I am aware of compilation problems with Microsoft Visual Studio (there even is an issue label for this kind of bug). I understand that even in 2016, complete C++11 support isn't there yet. But please also understand that I do not want to drop features or uglify the code just to make Microsoft's sub-standard compiler happy. The past has shown that there are ways to express the functionality such that the code compiles with the most recent MSVC - unfortunately, this is not the main objective of the project.
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