Shadowing in tupple overlay

[flaviojs]

How do you want shadowing to work? It's clear that files should shadow other files with the same path, but what about dirs?

Right now with a tupple overlay a "/a" file with prevent the "/a" dir ("/a/a" file) of another store from appearing in the entries iterator of "/", but you can still open the "/a/a" file so it's actually a partial shadow. (dir shadows the file if you reverse the order)

Personally, I would allow at most one file entry and one dir entry with the same name. This would mean one "a" file entry and one "a" dir entry from the entries iterator in the example above.

The initial design of this crate seemed file-centric, so I think it fits too. :) (dirs wouldn't be able to shadow files)