### What it does
Checks for usage of `_.map(_).flatten(_)` on `Iterator` and `Option`

### Why is this bad?
Readability, this can be written more concisely as
`_.flat_map(_)` for `Iterator` or `_.and_then(_)` for `Option`

### Example
```
let vec = vec![vec![1]];
let opt = Some(5);

vec.iter().map(|x| x.iter()).flatten();
opt.map(|x| Some(x * 2)).flatten();
```

Use instead:
```
vec.iter().flat_map(|x| x.iter());
opt.and_then(|x| Some(x * 2));
```