Store IStream instance owning ptrs in reporter instances

This ended up being a surprisingly large refactoring, motivated
by removing a `const_cast` from `Config`'s handling of reporter
streams, forced by previous commit.

src/catch2/interfaces/catch_interfaces_reporter.cpp

@@ -17,21 +17,25 @@
 #include <catch2/internal/catch_move_and_forward.hpp>
 
 #include <algorithm>
+#include <cassert>
 #include <iomanip>
 
 namespace Catch {
 
     ReporterConfig::ReporterConfig(
         IConfig const* _fullConfig,
-        IStream* _stream,
+        Detail::unique_ptr<IStream> _stream,
         ColourMode colourMode,
         std::map<std::string, std::string> customOptions ):
-        m_stream( _stream ),
+        m_stream( CATCH_MOVE(_stream) ),
         m_fullConfig( _fullConfig ),
         m_colourMode( colourMode ),
         m_customOptions( CATCH_MOVE( customOptions ) ) {}
 
-    IStream* ReporterConfig::stream() const { return m_stream; }
+    Detail::unique_ptr<IStream> ReporterConfig::takeStream() && {
+        assert( m_stream );
+        return CATCH_MOVE( m_stream );
+    }
     IConfig const * ReporterConfig::fullConfig() const { return m_fullConfig; }
     ColourMode ReporterConfig::colourMode() const { return m_colourMode; }