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/LibHTML/CSS/StyleProperties.cpp

@@ -109,7 +109,7 @@ void StyleProperties::load_font() const
 
     auto look_for_file = [](const StringView& expected_name) -> String {
         // TODO: handle font sizes properly?
-        CDirIterator it { "/res/fonts/", CDirIterator::Flags::SkipDots };
+        Core::DirIterator it { "/res/fonts/", Core::DirIterator::Flags::SkipDots };
         while (it.has_next()) {
             String name = it.next_path();
             ASSERT(name.ends_with(".font"));