Forbid name shadowing on comprehensions

The following expression is currently allowed [ref](https://github.com/0xPolygonMiden/air-script/blob/next/air-script-core/src/access.rs#L106):

```
[x + y + z for (x, y, z) in (x, 0..5, z[1..6])]
```

In the case above, the variable `x` with the iterable is shadowed with the yielded values `x`. This can be confusing at times, and may make sense to disallow that, requiring the user to be explicit and write something like:

```
[x_ + y + z for (x_, y, z) in (x, 0..5, z[1..6])]
```

The idea is to be explicit and hopefully prevent bugs.