Refactored stream related stuff

- simpler, polymorphic hierarchy-based, approach
- less bitty conditionals spread across the code
- all resolved up-front so now config class is immutable
(it had evolved the way it was and in need of a clean-up sweep for a long time)

include/internal/catch_impl.hpp

@@ -45,6 +45,7 @@
 namespace Catch {
     NonCopyable::~NonCopyable() {}
     IShared::~IShared() {}
+    IStream::~IStream() CATCH_NOEXCEPT {}
     StreamBufBase::~StreamBufBase() CATCH_NOEXCEPT {}
     IContext::~IContext() {}
     IResultCapture::~IResultCapture() {}