This simplifies the implementation of remotes. The only required interface is now `List` which is a simple one level directory list. Optionally remotes may implement `ListR` if they have an efficient way of doing a recursive list.
This is supported remotes which can do a recursive listing. It will use more memory. This is related to #1277 but doesn't fix that issue yet.
This is in preparation for removing the Lister code and replacing the fundamental operation in the Fs with listing a single directory.
