fixed_string: add string_view operator

fixed_string.hpp

@@ -32,6 +32,11 @@ namespace cruft {
         }
 
         constexpr operator char const* () const { return value; }
+        constexpr operator std::string_view () const { return value; }
+
+        constexpr char const*
+        c_str (void) const noexcept
+        { return value; }
     };