Auto-substream.

Detect if a stream has a parent, and if yes, automatically create a
substream.

Example:

    to create
        Root/
            A
            Aa
            Aaa
            B/
                Foo
                Bar

    all we have to do is to resolve the following paths:
        hoa://Test/Vfs/Root?type=directory
        hoa://Test/Vfs/Root/A
        hoa://Test/Vfs/Root/Aa
        hoa://Test/Vfs/Root/Aaa
        hoa://Test/Vfs/Root/B?type=directory
        hoa://Test/Vfs/Root/B/Foo
        hoa://Test/Vfs/Root/B/Bar