[boost] Removal of boost::basic_string_ref (in favor of basic_string_view)

I have just removed boost::basic_string_ref from the develop branch, and updated the specializations to use basic_string_view. Code that uses boost::string_ref (and wstring_ref) should continue to work as before. The basic_string_view class better matches what the standards committee is doing and has WAY more constexpr support. I'll be watching the testbots for results, but I believe that the only part of boost that was using basic_string_ref was Boost.Log, and that was switched over to basic_string_view in December. -- Marshall