Rust-itertools

- Add new adaptor `.merge_join_by` by srijs; a heterogeneous merge join for two ordered sequences.

- Iterator adaptors and iterators in itertools now use the same `must_use` reminder that the standard library adaptors do, by matematikaedit and bluss *“iterator adaptors are lazy and do nothing unless consumed”*.

- Faster `izip!()` by krdln
- `izip!()` is now a wrapper for repeated regular `.zip()` and a single `.map()`. This means it optimizes as well as the standard library `.zip()` it uses. **Note:** `multizip` and `izip!()` are now different! The former has a named type but the latter optimizes better.
- Faster `.unique()`
- `no_std` support, which is opt-in!
- Many lovable features are still there without std, like `izip!()` or `.format()` or `.merge()`, but not those that use collections.
- Trait bounds were required up front instead of just on the type: `group_by`'s `PartialEq` by Phlosioneer and `repeat_call`'s `FnMut`.
- Removed deprecated constructor `Zip::new` — use `izip!()` or `multizip()`

- Fix bug in `.cartesian_product()`'s fold (which only was visible for unfused iterators).

- Add specific `fold` implementations for `.cartesian_product()` and `cons_tuples()`, which improves their performance in fold, foreach, and iterator consumers derived from them.

- Add iterator adaptor `.positions(predicate)` by tmccombs