Represent the Tahoe-LAFS idea of a directory using Haskell types

Tahoe-LAFS includes a representation for a directory - a collection of "regular" and "directory" entries with names and other metadata. To interact with these directories (mainly to render information about them into the mobile app read-only view) we need a Haskell representation. The Haskell representation should be able to represent all of the same directories as the Python representation.

Specifically, we want a somewhat abstract representation - some type that supports operations like "list the names of the entries" and "retrieve the contents associated with entry E", etc.