Forbid name shadowing on comprehensions

[hackaugusto]

The following expression is currently allowed ref:

[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.