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

[sehe]

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