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.

Games/Snake/SnakeGame.cpp

@@ -102,7 +102,7 @@ Rect SnakeGame::high_score_rect() const
     return { 2, height() - font().glyph_height() - 2, high_score_width, font().glyph_height() };
 }
 
-void SnakeGame::timer_event(CTimerEvent&)
+void SnakeGame::timer_event(Core::TimerEvent&)
 {
     Vector<Coordinate> dirty_cells;