Add .mli for FS operations

lib/fs.mli

@@ -0,0 +1,43 @@
+(** A definition of a file tree. *)
+type tree =
+  | File of string
+  | Dir of string * tree array
+
+(** Return the name of a given tree node. *)
+val file_name : tree -> string
+
+(** Read directory contents recursively from a given file path. *)
+val read_tree : string -> tree
+
+(** A cursor for files inside a single directory. *)
+type cursor = {
+  pos : int;
+  files : tree array;
+}
+
+(** Return the currently selected file in file cursor. *)
+val file_at : cursor -> tree option
+
+(** A file tree zipper that allows to traverse the tree in four directions. *)
+type zipper = {
+  parents : cursor list;
+  current : cursor;
+}
+
+(** Constructs a zipper from the contents of a given directory. *)
+val zip_it : tree array -> zipper
+
+(** Returns the list of parents names in reverse order. *)
+val zipper_parents : zipper -> string list
+
+(** Move to the next file within the same directory. Cycles. *)
+val go_down : zipper -> zipper
+
+(** Move to the previous file within the same directory. Cycles. *)
+val go_up : zipper -> zipper
+
+(** Move to the directory under the current cursor. Doesn't move inside files or empty directories. *)
+val go_next : zipper -> zipper
+
+(** Move to the parent directory. *)
+val go_back : zipper -> zipper