Rust-itertools

- Add k-ary merge adaptor `.kmerge()`
- Fix a bug in `.islice()` with ranges `a..b` where a `> b`.

- Implement `Clone`, `Debug` for `Linspace`

- Add function `diff_with()` that compares two iterators
- Add `.combinations_n()`, an n-ary combinations iterator
- Add methods `PutBack::with_value` and `PutBack::into_parts`.

- Add method `.sorted()`
- Add module `itertools::free` with free function variants of common iterator adaptors and methods. For example `enumerate(iterable)`, `rev(iterable)`, and so on.

- Add `.flatten()`

- Allow composing `ZipSlices` with itself