src/pxar/fuse.rs: impl lookup callback for fuse

src/pxar/fuse.rs: impl lookup callback for fuse

The lookup call checks if the given filename is found in the directory referenced
by the i-node by calclulating the filenames hash and looking it up in the
directories goodbye table.
If found, the entries parameters are returned.
In order to be able to lookup the parent offset by a given file offset in the
readdir callback, this also stores the corresponding values in a HashMap.

Signed-off-by: Christian Ebner <c.ebner@proxmox.com>