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.

projects/SelfTest/MatchersTests.cpp

@@ -8,6 +8,8 @@
 
 #include "catch.hpp"
 
+#include <sstream>
+
 #ifdef __clang__
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wweak-vtables"

projects/SelfTest/MiscTests.cpp

@@ -18,6 +18,7 @@
 #include <iostream>
 #include <cerrno>
 #include <limits>
+#include <sstream>
 
 TEST_CASE( "random SECTION tests", "[.][sections][failing]" ) {
     int a = 1;

projects/SelfTest/TrickyTests.cpp

@@ -15,10 +15,11 @@
                                  // that is triggered when compiling as Win32|Release
 #endif
 
-#include <stdio.h>
-
 #include "catch.hpp"
 
+#include <stdio.h>
+#include <sstream>
+
 namespace Catch {
     std::string toString( const std::pair<int, int>& value ) {
         std::ostringstream oss;