### What it does
Warns when using `push_str`/`insert_str` with a single-character string literal
where `push`/`insert` with a `char` would work fine.

### Why is this bad?
It's less clear that we are pushing a single character.

### Example
```
string.insert_str(0, "R");
string.push_str("R");
```

Use instead:
```
string.insert(0, 'R');
string.push('R');
```