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/man.cpp

@@ -86,10 +86,10 @@ int main(int argc, char* argv[])
         }
     }
 
-    auto file = CFile::construct();
+    auto file = Core::File::construct();
     file->set_filename(make_path(section));
 
-    if (!file->open(CIODevice::OpenMode::ReadOnly)) {
+    if (!file->open(Core::IODevice::OpenMode::ReadOnly)) {
         perror("Failed to open man page file");
         exit(1);
     }