Rust-itertools

- Use `slice::iter` instead of `into_iter` to avoid future breakage (378, by LukasKalbertodt)

- Added a [`.exactly_one()`](https://docs.rs/itertools/0.8.1/itertools/trait.Itertools.html#method.exactly_one) iterator method that, on success, extracts the single value of an iterator ; by Xaeroxe
- Added combinatory iterator adaptors:
- [`.permutations(k)`](https://docs.rs/itertools/0.8.1/itertools/trait.Itertools.html#method.permutations):

`[0, 1, 2].iter().permutations(2)` yields

rust
[
vec![0, 1],
vec![0, 2],
vec![1, 0],
vec![1, 2],
vec![2, 0],
vec![2, 1],
]


; by tobz1000

- [`.combinations_with_replacement(k)`](https://docs.rs/itertools/0.8.1/itertools/trait.Itertools.html#method.combinations_with_replacement):

`[0, 1, 2].iter().combinations_with_replacement(2)` yields

rust
[
vec![0, 0],
vec![0, 1],
vec![0, 2],
vec![1, 1],
vec![1, 2],
vec![2, 2],
]


; by tommilligan

- For reference, these methods join the already existing [`.combinations(k)`](https://docs.rs/itertools/0.8.1/itertools/trait.Itertools.html#method.combinations):

`[0, 1, 2].iter().combinations(2)` yields

rust
[
vec![0, 1],
vec![0, 2],
vec![1, 2],
]


- Improved the performance of [`.fold()`](https://docs.rs/itertools/0.8.1/itertools/trait.Itertools.html#method.fold)-based internal iteration for the [`.intersperse()`](https://docs.rs/itertools/0.8.1/itertools/trait.Itertools.html#method.intersperse) iterator ; by jswrenn
- Added [`.dedup_by()`](https://docs.rs/itertools/0.8.1/itertools/trait.Itertools.html#method.dedup_by), [`.merge_by()`](https://docs.rs/itertools/0.8.1/itertools/trait.Itertools.html#method.merge_by) and [`.kmerge_by()`](https://docs.rs/itertools/0.8.1/itertools/trait.Itertools.html#method.kmerge_by) adaptors that work like [`.dedup()`](https://docs.rs/itertools/0.8.1/itertools/trait.Itertools.html#method.dedup), [`.merge()`](https://docs.rs/itertools/0.8.1/itertools/trait.Itertools.html#method.merge) and [`.kmerge()`](https://docs.rs/itertools/0.8.1/itertools/trait.Itertools.html#method.kmerge), but taking an additional custom comparison closure parameter. ; by phimuemue
- Improved the performance of [`.all_equal()`](https://docs.rs/itertools/0.8.1/itertools/trait.Itertools.html#method.all_equal) ; by fyrchik
- Loosened the bounds on [`.partition_map()`](https://docs.rs/itertools/0.8.1/itertools/trait.Itertools.html#method.partition_map) to take just a `FnMut` closure rather than a `Fn` closure, and made its implementation use internal iteration for better performance ; by danielhenrymantilla
- Added convenience methods to [`EitherOrBoth`](https://docs.rs/itertools/0.8.1/itertools/enum.EitherOrBoth.html) elements yielded from the [`.zip_longest()`](https://docs.rs/itertools/0.8.1/itertools/trait.Itertools.html#method.zip_longest) iterator adaptor ; by Avi-D-coder
- Added [`.sum1()`](https://docs.rs/itertools/0.8.1/itertools/trait.Itertools.html#method.sum1) and [`.product1()`](https://docs.rs/itertools/0.8.1/itertools/trait.Itertools.html#method.product1) iterator methods that respectively try to return the sum and the product of the elements of an iterator **when it is not empty**, otherwise they return `None` ; by Emerentius

- Added new adaptor `.map_into()` for conversions using `Into` by vorner
- Improved `Itertools` docs by JohnHeitmann
- The return type of `.sorted_by_by_key()` is now an iterator, not a Vec.
- The return type of the `izip!(x, y)` macro with exactly two arguments is now the usual `Iterator::zip`.
- Remove `.flatten()` in favour of std's `.flatten()`
- Deprecate `.foreach()` in favour of std's `.for_each()`
- Deprecate `.step()` in favour of std's `.step_by()`
- Deprecate `repeat_call` in favour of std's `repeat_with`
- Deprecate `.fold_while()` in favour of std's `.try_fold()`
- Require Rust 1.24 as minimal version.

- Add convenience methods to `EitherOrBoth`, making it more similar to `Option` and `Either` by jethrogb

- No changes.

- New inclusion policy: See the readme about suggesting features for std before accepting them in itertools.
- The `FoldWhile` type now implements `Eq` and `PartialEq` by jturner314