This replaces only using flat filenames, to better test nested file
behaviors.
Fixed inmemory/mfs.go and filesystem/driver.go after finding bugs with
the new tests and test behavior.
This change started out as simply updating the existing inmemory driver to
implement the new Stat call. After struggling with the map based
implementation, it has been refactored to be a tree-based implementation.
This process has exposed a few missing error cases in the StorageDriver API
that should be addressed in the coming weeks.
