Refactor implementation of case-insensitivity in tags

By not materializing the lower cased tags ahead of time, we
save allocations at the cost of worsened performance when comparing
two tags.

Since there are rarely many tags, and commonly they are not
compared even if present, this is almost always a win. The new
implementation also improves the robustness of the code
responsible for handling tags in a case-insensitive manner.

examples/210-Evt-EventListeners.cpp

@@ -22,7 +22,7 @@ std::string ws(int const level) {
 }
 
 std::ostream& operator<<(std::ostream& out, Catch::Tag t) {
-    return out << "original: " << t.original << "lower cased: " << t.lowerCased;
+    return out << "original: " << t.original;
 }
 
 template< typename T >