Make StringRef's operator std::string explicit

This way it is explicit when there is a `StringRef` -> `std::string`
conversion and makes it easier to look for allocations that could
be avoided.

Doing this has already removed one allocation per registered test
case, as there was a completely pointless `StringRef` -> `std::string`
conversion when parsing tags of a test case.

include/internal/catch_test_case_registry_impl.cpp

@@ -105,7 +105,7 @@ namespace Catch {
     }
 
     std::string extractClassName( StringRef const& classOrQualifiedMethodName ) {
-        std::string className = classOrQualifiedMethodName;
+        std::string className(classOrQualifiedMethodName);
         if( startsWith( className, '&' ) )
         {
             std::size_t lastColons = className.rfind( "::" );