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.

Userland/pro.cpp

@@ -45,7 +45,7 @@ int main(int argc, char** argv)
         return 1;
     }
 
-    CEventLoop loop;
+    Core::EventLoop loop;
     auto protocol_client = LibProtocol::Client::construct();
 
     auto download = protocol_client->start_download(url.to_string());