Support vectors in to_string

Currently, a vector would have to be wrapped in another struct that implements the `ToString` trait.
Ideally, we want to be able to do this:

```rust
#[table_row]
struct Row {
id: u32,
privileges: Vec
}
```

And when calling the `to_string` method, this would output:
```
+----+------------+
| id | privileges |
+====+============+
| 1 | admin |
| | user |
+----+------------+
| 2 | user |
+----+------------+
```