Cleanup for performance reasons

* Eliminated some copies
* Made makeTestCase fit into 4 arguments -- avoids spills on Win64
* Made string literals into StringRef literals

include/internal/catch_test_case_registry_impl.cpp

@@ -96,7 +96,7 @@ namespace Catch {
         m_testAsFunction();
     }
 
-    std::string extractClassName( std::string const& classOrQualifiedMethodName ) {
+    std::string extractClassName( StringRef const& classOrQualifiedMethodName ) {
         std::string className = classOrQualifiedMethodName;
         if( startsWith( className, '&' ) )
         {