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_test_case_info.cpp

@@ -15,6 +15,7 @@
 #include <cctype>
 #include <exception>
 #include <algorithm>
+#include <sstream>
 
 namespace Catch {