Use core string_view

fix #2417

This improves inter-conversion between string_view implementations. Some observable differences for users:
 - core::string_view no longer supports the .to_string() or .clear() extensions from Utility
 - code that relied on .max_size() returning .size(), needs to be fixed to .size() instead
 - remove_suffix() and remove_prefix() were more lenient than the standard specs; be sure you don't rely on it clamping the argument to valid range
 - BOOST_NO_CXX11_EXPLICIT_CONVERSION_OPERATORS no longer suppresses conversions to std::string
 - core::string_view adds .contains() and various bugs fixed

include/boost/beast/core/string_type.hpp

@@ -15,7 +15,7 @@
 #if defined(BOOST_BEAST_USE_STD_STRING_VIEW)
 #include <string_view>
 #else
-#include <boost/utility/string_view.hpp>
+#include <boost/core/detail/string_view.hpp>
 #endif
 
 namespace boost {
@@ -23,7 +23,7 @@ namespace beast {
 
 #if BOOST_BEAST_DOXYGEN || ! defined(BOOST_BEAST_USE_STD_STRING_VIEW)
 /// The type of string view used by the library
-using string_view = boost::string_view;
+using string_view = boost::core::string_view;
 
 /// The type of `basic_string_view` used by the library
 template<class CharT, class Traits>