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.

Libraries/LibThread/Thread.cpp

@@ -29,7 +29,7 @@
 #include <unistd.h>
 
 LibThread::Thread::Thread(Function<int()> action, StringView thread_name)
-    : CObject(nullptr)
+    : Core::Object(nullptr)
     , m_action(move(action))
     , m_thread_name(thread_name.is_null() ? "" : thread_name)
 {