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/ps.cpp

@@ -37,7 +37,7 @@ int main(int argc, char** argv)
 
     printf("PID TPG PGP SID UID  STATE        PPID NSCHED     FDS TTY   NAME\n");
 
-    auto all_processes = CProcessStatisticsReader::get_all();
+    auto all_processes = Core::ProcessStatisticsReader::get_all();
 
     for (const auto& it : all_processes) {
         const auto& proc = it.value;