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/reporters/catch_reporter_teamcity.hpp

@@ -14,6 +14,8 @@
 // file can be distributed as a single header that works with the main
 // Catch single header.
 
+#include "../internal/catch_enforce.h"
+
 #include <cstring>
 
 #ifdef __clang__
@@ -69,9 +71,9 @@ namespace Catch {
             AssertionResult const& result = assertionStats.assertionResult;
             if( !result.isOk() ) {
 
-                std::ostringstream msg;
+                ReusableStringStream msg;
                 if( !m_headerPrintedForThisSection )
-                    printSectionHeader( msg );
+                    printSectionHeader( msg.get() );
                 m_headerPrintedForThisSection = true;
 
                 msg << result.getSourceInfo() << "\n";