Introduced ReusableStringStream and removed all uses of std::ostringstream from the main path

ReusableStringStream holds a std::ostringstream internally, but only exposes the ostream interface.
It caches a pool of ostringstreams in a vector which is currently global, but will be made thread-local.

Altogether this should enable both runtime and compile-time benefits. although more work is needed to realise the compile time opportunities.

include/internal/catch_console_colour.cpp

@@ -23,6 +23,8 @@
 #include "catch_debugger.h"
 #include "catch_windows_h_proxy.h"
 
+#include <sstream>
+
 namespace Catch {
     namespace {