Add children_iter_mut()

[niklasdewally]

Description

A common pattern is to do

let mut children = expr.children();
for child in children.iter_mut() {
  // do some stuff
}
let expr = expr.with_children(children);

It would be nice to do :

for child in expr.iter_mut() {
  // do some stuff
}

In a sense this is just descend(), but it links with the rest of Rust's Iterator API, instead of just being a map.

More generally, we could provide children_iter(), universe_iter(), children_iter_mut(), universe_iter_mut() to turn all the things into iterators.

Questions:

• Exactly which iter functions do we provide and what do we call them?

• How should we traverse for universe_iter()? Should this be like transform()?

To-do

• Implement children_iter(), universe_iter()
• Implement children_bi_iter(), universe_bi_iter()
• Implement children_iter_mut(), universe_iter_mut()
• Implement children_bi_iter_mut(), universe_bi_iter_mut()