`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()?;
```
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