Rust-itertools

- Fixup on top of the workaround in 0.4.14. A function in `itertools::free` was removed by mistake and now it is added back again.

- Workaround an upstream regression in a Rust nightly build that broke compilation of of `itertools::free::{interleave, merge}`

- Add `.minmax()` and `.minmax_by_key()`, iterator methods for finding both minimum and maximum in one scan.
- Add `.format_default()`, a simpler version of `.format()` (lazy formatting for iterators).

- Add `.zip_eq()`, an adaptor like `.zip()` except it ensures iterators of inequal length don't pass silently (instead it panics).
- Add `.fold_while()`, an iterator method that is a fold that can short-circuit.
- Add `.partition_map()`, an iterator method that can separate elements into two collections.

- Add `.get()` for `Stride{,Mut}` and `.get_mut()` for `StrideMut`

- Improve performance of `.kmerge()`