Zipping iterators

[lukaszcz]

We should have iterators for2, map2, etc., which are equivalent to for, map, etc., on zipped lists / data structures.

For example,

for2 (acc := a) (x in lst1; y in lst2) {
  f acc x y
}

would be equivalent to

for (acc := a) (x, y in zip lst1 lst2) {
  f acc x y
}

The first is better both for readability / understandability for newcomers and efficiency (no intermediate list created).