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/shutdown.cpp

@@ -32,7 +32,7 @@ int main(int argc, char** argv)
 {
     bool now = false;
 
-    CArgsParser args_parser;
+    Core::ArgsParser args_parser;
     args_parser.add_option(now, "Shut down now", "now", 'n');
     args_parser.parse(argc, argv);