Constexpr basic_string_view

### Checklist

- [x] I did not find a duplicate of this feature request in the Github Issues section.

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### Description
The bpstd::basic_string_view uses std::char_traits as a background. This produces a error in a constexpr context:
`error: call to non-'constexpr' function 'static std::size_t std::char_traits::length(const char_type*)'`.
This is [correspond](https://en.cppreference.com/w/cpp/string/char_traits/length) to the standard because char_traits::length is marked constexpr since C++17 only.

P.S. Idk whether it's a bug or a feature request.