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_run_context.h

@@ -32,13 +32,12 @@ namespace Catch {
 
     public:
         StreamRedirect(std::ostream& stream, std::string& targetString);
-
         ~StreamRedirect();
 
     private:
         std::ostream& m_stream;
         std::streambuf* m_prevBuf;
-        std::ostringstream m_oss;
+        ReusableStringStream m_oss;
         std::string& m_targetString;
     };
 
@@ -52,7 +51,7 @@ namespace Catch {
     private:
         std::streambuf* m_cerrBuf;
         std::streambuf* m_clogBuf;
-        std::ostringstream m_oss;
+        ReusableStringStream m_oss;
         std::string& m_targetString;
     };