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.

tests/ExtraTests/X23-CasingInReporterNames.cpp

@@ -19,11 +19,12 @@
 #include <catch2/reporters/catch_reporter_registrars.hpp>
 
 #include <iostream>
+#include <utility>
 
 class TestReporter : public Catch::StreamingReporterBase {
 public:
-    TestReporter(Catch::ReporterConfig const& _config):
-        StreamingReporterBase(_config) {
+    TestReporter(Catch::ReporterConfig&& _config):
+        StreamingReporterBase(std::move(_config)) {
         std::cout << "TestReporter constructed\n";
     }