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/LibGUI/GFontDatabase.cpp

@@ -43,7 +43,7 @@ GFontDatabase& GFontDatabase::the()
 
 GFontDatabase::GFontDatabase()
 {
-    CDirIterator di("/res/fonts", CDirIterator::SkipDots);
+    Core::DirIterator di("/res/fonts", Core::DirIterator::SkipDots);
     if (di.has_error()) {
         fprintf(stderr, "CDirIterator: %s\n", di.error_string());
         exit(1);