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.

Applications/ChanViewer/ThreadCatalogModel.cpp

@@ -52,7 +52,7 @@ void ThreadCatalogModel::set_board(const String& board)
 
 void ThreadCatalogModel::update()
 {
-    CHttpRequest request;
+    Core::HttpRequest request;
     request.set_url(String::format("http://a.4cdn.org/%s/catalog.json", m_board.characters()));
 
     if (m_pending_job)