[boost] Re: utility::string_view and core::detail::string_view

This amazing discussion reminded me that I had some string_view benchmarks sitting on my drive since forever, and I remembered that there was even an issue about it which I never addressed: https://github.com/boostorg/core/issues/114 So I added them now, four years too late. https://github.com/boostorg/core/commit/b7fdbf41a7523e872960b56108c26db1cb1d... In addition to adding them I also ran them, and I observed something that might be of interest: Microsoft's std::string_view hasn't stood still and, in VS2022, has AVX2 optimized implementations of (at least) find_first_of, which makes it twice as fast as core::string_view.