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_message.h

@@ -9,9 +9,9 @@
 #define TWOBLUECUBES_CATCH_MESSAGE_H_INCLUDED
 
 #include <string>
-#include <sstream>
 #include "catch_result_type.h"
 #include "catch_common.h"
+#include "catch_stream.h"
 
 namespace Catch {
 
@@ -40,8 +40,7 @@ namespace Catch {
             return *this;
         }
 
-        // !TBD reuse a global/ thread-local stream
-        std::ostringstream m_stream;
+        ReusableStringStream m_stream;
     };
 
     struct MessageBuilder : MessageStream {