Unify IReporterRegistry and ReporterRegistry

To keep the compilation firewall effect, the implementations
are hidden behind a PIMPL. In this case it is probably not
worth it, but we can inline it later if needed.

src/catch2/catch_registry_hub.cpp

@@ -20,6 +20,7 @@
 #include <catch2/internal/catch_noncopyable.hpp>
 #include <catch2/interfaces/catch_interfaces_reporter_factory.hpp>
 #include <catch2/internal/catch_move_and_forward.hpp>
+#include <catch2/internal/catch_reporter_registry.hpp>
 
 namespace Catch {
 
@@ -31,7 +32,7 @@ namespace Catch {
 
         public: // IRegistryHub
             RegistryHub() = default;
-            IReporterRegistry const& getReporterRegistry() const override {
+            ReporterRegistry const& getReporterRegistry() const override {
                 return m_reporterRegistry;
             }
             ITestCaseRegistry const& getTestCaseRegistry() const override {