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/AudioServer/ASClientConnection.h

@@ -37,7 +37,7 @@ class ASClientConnection final : public IClientConnection<AudioServerEndpoint>
     , public AudioServerEndpoint {
     C_OBJECT(ASClientConnection)
 public:
-    explicit ASClientConnection(CLocalSocket&, int client_id, ASMixer& mixer);
+    explicit ASClientConnection(Core::LocalSocket&, int client_id, ASMixer& mixer);
     ~ASClientConnection() override;
 
     void did_finish_playing_buffer(Badge<ASBufferQueue>, int buffer_id);