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.

Servers/TelnetServer/main.cpp

@@ -106,8 +106,8 @@ static void run_command(int ptm_fd, String command)
 
 int main(int argc, char** argv)
 {
-    CEventLoop event_loop;
-    auto server = CTCPServer::construct();
+    Core::EventLoop event_loop;
+    auto server = Core::TCPServer::construct();
 
     int opt;
     u16 port = 23;