LibCore: Put all classes in the Core namespace and remove the leading C

I've been wanting to do this for a long time. It's time we start being
consistent about how this stuff works.

The new convention is:

- "LibFoo" is a userspace library that provides the "Foo" namespace.

That's it :^) This was pretty tedious to convert and I didn't even
start on LibGUI yet. But it's coming up next.

Applications/SystemMonitor/main.cpp

@@ -146,7 +146,7 @@ int main(int argc, char** argv)
     toolbar->set_has_frame(false);
     auto process_table_view = ProcessTableView::construct(process_table_container);
 
-    auto refresh_timer = CTimer::construct(1000, [&] {
+    auto refresh_timer = Core::Timer::construct(1000, [&] {
         process_table_view->refresh();
         if (auto* memory_stats_widget = MemoryStatsWidget::the())
             memory_stats_widget->refresh();