Refactored stream related stuff

- simpler, polymorphic hierarchy-based, approach
- less bitty conditionals spread across the code
- all resolved up-front so now config class is immutable
(it had evolved the way it was and in need of a clean-up sweep for a long time)

projects/SelfTest/SurrogateCpps/catch_stream.cpp

@@ -1,2 +1,3 @@
 // This file is only here to verify (to the extent possible) the self sufficiency of the header
+#include "catch_suppress_warnings.h"
 #include "catch_stream.h"