Pass other StringRef arguments by value instead of by-ref

Apart from being clearer, it also improves the overall codesize
of the implementation library, and should improve the performance
as well, by removing one level of indirection.

src/catch2/internal/catch_run_context.cpp

@@ -519,7 +519,7 @@ namespace Catch {
     void RunContext::handleMessage(
             AssertionInfo const& info,
             ResultWas::OfType resultType,
-            StringRef const& message,
+            StringRef message,
             AssertionReaction& reaction
     ) {
         m_reporter->assertionStarting( info );