Added StringRef constructor that captures string literal size at compile time

include/internal/catch_stringref.cpp

@@ -16,6 +16,11 @@
 #include <ostream>
 
 namespace Catch {
+    auto StringRef::fromRaw( char const* rawChars ) -> StringRef {
+        return rawChars
+            ? StringRef( rawChars,static_cast<StringRef::size_type>(std::strlen(rawChars) ) )
+            : StringRef();
+    }
 
     StringRef::operator std::string() const {
         return std::string( m_start, m_size );