Make IStream::stream non-const

This way it makes much more sense from logically-const point
of view, and also means that concrete implementations don't
have to always have a `mutable` keyword on the stream member.

tests/SelfTest/IntrospectiveTests/ColourImpl.tests.cpp

@@ -21,9 +21,9 @@ namespace {
     };
 
     class TestStringStream : public Catch::IStream {
-        mutable std::stringstream m_stream;
+        std::stringstream m_stream;
     public:
-        std::ostream& stream() const override {
+        std::ostream& stream() override {
             return m_stream;
         }