Add BOOST_BEAST_USE_STD_STRING_VIEW:

fix #1133, fix #1241

When the macro BOOST_BEAST_USE_STD_STRING_VIEW is defined,
Beast will use std::string_view instead of boost::string_view.

The name boost::beast::string_view is a type alias for the
chosen view type.

include/boost/beast/core/string.hpp

@@ -12,19 +12,35 @@
 
 #include <boost/beast/core/detail/config.hpp>
 #include <boost/version.hpp>
+
+#if defined(BOOST_BEAST_USE_STD_STRING_VIEW)
+#include <string_view>
+#else
 #include <boost/utility/string_view.hpp>
+#endif
+
 #include <algorithm>
 
 namespace boost {
 namespace beast {
 
-/// The type of string view used by the library
-using string_view = boost::string_view;
+#if defined(BOOST_BEAST_USE_STD_STRING_VIEW)
+  /// The type of string view used by the library
+  using string_view = std::string_view;
 
-/// The type of basic string view used by the library
-template<class CharT, class Traits>
-using basic_string_view =
-    boost::basic_string_view<CharT, Traits>;
+  /// The type of basic string view used by the library
+  template<class CharT, class Traits>
+  using basic_string_view =
+      std::basic_string_view<CharT, Traits>;
+#else
+  /// The type of string view used by the library
+  using string_view = boost::string_view;
+
+  /// The type of basic string view used by the library
+  template<class CharT, class Traits>
+  using basic_string_view =
+      boost::basic_string_view<CharT, Traits>;
+#endif
 
 namespace detail {