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.

Userland/killall.cpp

@@ -39,7 +39,7 @@ static void print_usage_and_exit()
 
 static int kill_all(const String& process_name, const unsigned signum)
 {
-    auto processes = CProcessStatisticsReader().get_all();
+    auto processes = Core::ProcessStatisticsReader().get_all();
 
     for (auto& it : processes) {
         if (it.value.name == process_name) {