Rust-itertools

- Add iterator adaptor `.with_position()`
- Fix multipeek's performance for long peeks by using `VecDeque`.

- Add `.map_results()`

- Many more adaptors now implement `Debug`
- Add free function constructor `repeat_n`. `RepeatN::new` is now deprecated.

- Add infinite generator function `iterate`, that takes a seed and a closure.

- Special-cased `.fold()` for flatten and put back. `.foreach()` now uses fold on the iterator, to pick up any iterator specific loop implementation.
- `.combinations(n)` asserts up front that `n != 0`, instead of running into an error on the second iterator element.

- Add `.tuples::<T>()` that iterates by two, three or four elements at a time (where `T` is a tuple type).
- Add `.tuple_windows::<T>()` that iterates using a window of the two, three or four most recent elements.
- Add `.next_tuple::<T>()` method, that picks the next two, three or four elements in one go.
- `.interleave()` now has an accurate size hint.