Auto merge of #8717 - Alexendoo:manual-split-once-manual-iter, r=dswij,xFrednet

`manual_split_once`: lint manual iteration of `SplitN`

changelog: `manual_split_once`: lint manual iteration of `SplitN`

Now lints:

```rust
let mut iter = "a.b.c".splitn(2, '.');
let first = iter.next().unwrap();
let second = iter.next().unwrap();

let mut iter = "a.b.c".splitn(2, '.');
let first = iter.next()?;
let second = iter.next()?;

let mut iter = "a.b.c".rsplitn(2, '.');
let first = iter.next().unwrap();
let second = iter.next().unwrap();

let mut iter = "a.b.c".rsplitn(2, '.');
let first = iter.next()?;
let second = iter.next()?;
```

It suggests (minus leftover whitespace):

```rust
let (first, second) = "a.b.c".split_once('.').unwrap();

let (first, second) = "a.b.c".split_once('.')?;

let (second, first) = "a.b.c".rsplit_once('.').unwrap();

let (second, first) = "a.b.c".rsplit_once('.')?;
```

Currently only lints if the statements are next to each other, as detecting the various kinds of shadowing was tricky, so the following won't lint

```rust
let mut iter = "a.b.c".splitn(2, '.');
let something_else = 1;
let first = iter.next()?;
let second = iter.next()?;
```

0 files changed, 0 insertions, 0 deletions