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/Help/ManualSectionNode.cpp

@@ -41,7 +41,7 @@ void ManualSectionNode::reify_if_needed() const
         return;
     m_reified = true;
 
-    CDirIterator dir_iter { path(), CDirIterator::Flags::SkipDots };
+    Core::DirIterator dir_iter { path(), Core::DirIterator::Flags::SkipDots };
 
     while (dir_iter.has_next()) {
         FileSystemPath file_path(dir_iter.next_path());