Implement while/for loops in MLIR

[rengolin]

With the dialect effort starting up, it's the right time to start using Verona dialect on the MLIR lowering, at least for the high level structure.

In Verona, for loops are syntactic sugar for while loops in the following way:

for (a in x) {
   foo(a);
}

equals:

{
  var $iter = x;
  while ($iter.has_value())
  {
    a = $iter.apply();
    $iter.next();
    foo(a);
  }
}

[rengolin]

In the end, this did not bring the benefits we were expecting, so I'll revert the patch creating loop regions, and continue working on loops as control flow. This will remove the new operations (loop_exit, loop_return, continue, break) and the region based approach.