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/ProtocolServer/PSClientConnection.cpp

@@ -32,7 +32,7 @@
 
 static HashMap<int, RefPtr<PSClientConnection>> s_connections;
 
-PSClientConnection::PSClientConnection(CLocalSocket& socket, int client_id)
+PSClientConnection::PSClientConnection(Core::LocalSocket& socket, int client_id)
     : IClientConnection(*this, socket, client_id)
 {
     s_connections.set(client_id, *this);