Add an iterator over Node's children and replace usages of Node.foreach_child()

Closes #229 (closed).

I added Node.children() which returns an iterator over Node's children and replaced existing usages of Node.foreach_child() with iteration.

I decided to call it .children() as opposed to .iter_children() since it's common in Rust (see for instance str.chars(), str.bytes(), slice.chunks()).

I had to move the comment on an unrelated line because it was making it too long (104 characters) and rustfmt in the CI was complaining.