package plebeia

File system over Plebeia tree, with a "tree" interface.

Here, a `tree` is a subtree of Plebeia tree. This is different from the `cursor` of `Fs` which is a zipper pointer to a tree.

Accesses to the tree items are by the absolute path names from the root of the tree. Fs_tree optimizes contiguous file accesses in the same directories using zipper internally.