Fix char_traits deprecation warning

[colbychaskell]

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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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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[coveralls]

coverage: 100.0%. remained the same when pulling 543158bbf3efe123d59832e26a89ebf63c9e5eb7 on colbychaskell:develop into f56c6e2e30241b9245161a86ae9fecf6543bf411 on nlohmann:develop.

[github-actions[bot]]

🔴 Amalgamation check failed! 🔴

The source code has not been amalgamated. @colbychaskell Please read and follow the Contribution Guidelines.

[github-actions[bot]]

🔴 Amalgamation check failed! 🔴

The source code has not been amalgamated. @colbychaskell Please read and follow the Contribution Guidelines.

[colbychaskell]

Pushed a new commit that handles this a lot more cleanly. Will fix check errors soon

[github-actions[bot]]

🔴 Amalgamation check failed! 🔴

The source code has not been amalgamated. @colbychaskell Please read and follow the Contribution Guidelines.

[colbychaskell]

@nlohmann Sorry for the delay, this is ready to be reviewed when you get a chance!

[colbychaskell]

@nlohmann it looks like something unrelated to these changes is breaking in the CI compiler tests as it's trying to get CMake and Ninja.

Would this be related to recent CI fixes? The checks passed successfully with the same changes about a week ago.

[nlohmann]

No, unfortunately this is unrelated. I am getting so tired of fixing CI issues rather than improving the library. I'll check during the weekend.

[colbychaskell]

The failing checks are an unrelated CI issue that will be fixed in #4215

[nlohmann]

The failing checks are an unrelated CI issue that will be fixed in #4215

4215 is merged.

[nlohmann]

Please rebase to the develop branch. This should fix the CI. I would like to take this as last item for the upcoming release.

[colbychaskell]

@nlohmann rebased and good to go now!

[nlohmann]

Thanks a lot!