boost::core::string_view doesn't support non-standard character traits

`boost::core::basic_string_view` replicates the standard `std::string_view` interface. However, it lacks support for the char_trait template argument wholesale.

While this omission is defensible, it might be a reason stopping people from using `core::basic_string_view` in their code.

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¹ except for conversion from/to std::basic_string, for obvious reasons