Rust Zipper crate for Lists/Sequenceable Data

Zippers are data structures representing partial iteration through some structure. they capture a hole in the data structure -- the focused element -- along with the required context to move through the structure. this crate provides a Zipper over sequenceable data, turning an `Iterator` over the data into a `Zipper` that allows moving forwards and backwards through the data.

additionally, the provided zipper turns the initial sequence into a ring, with the last element of the initial sequence connecting to the first element of that same sequence. this makes the data structure a natural fit for navigation through UI elements, where iteration should never abruptly end, and where context needs to be preserved. examples include tabs in a web browser or windows in a tiling window manager.

usage example:

use list_zipper::*;
use SequenceDirection::*;

let mut zipper = (0..10)
  .into_iter()
  .collect::<Zipper<_>>();

// focus the 5th element of the sequence
zipper.refocus(|i| *i == 5);

// print out the whole zipper to show the iteration order:
assert_eq!(zipper.to_string(), "[5, 6, 7, 8, 9, 0, 1, 2, 3, 4]");

// focus the 6th element
zipper.step_forwards();
assert_eq!(zipper.focus(), Some(&6));

// focus the 4th element
zipper.step_backwards().step_backwards();
assert_eq!(zipper.focus(), Some(&4));

// focus the 9th element
zipper
  .step_backwards()
  .step_backwards()
  .step_backwards()
  .step_backwards()
  .step_backwards();
assert_eq!(zipper.focus(), Some(&9));

// focus the 0th element
zipper.step_forwards();
assert_eq!(zipper.focus(), Some(&0));