Nested loops

[dtolnay]

The _loop_indexes mechanism makes it seem like nested loops are intended to be supported, but I haven't figured it out. Here is what I tried:

{% for y in names %}
  {% for x in y %}
    x = {{ x }}
  {% endfor %}
{% endfor %}

#[derive(Template)]
#[template(path = "test")]
struct Test<'a> {
    names: Vec<Vec<&'a str>>,
}

error[E0277]: the trait bound `&&std::vec::Vec<&str>: std::iter::Iterator` is not satisfied
 --> src/main.rs:7:10
  |
7 | #[derive(Template)]
  |          ^^^^^^^^ the trait `std::iter::Iterator` is not implemented for `&&std::vec::Vec<&str>`
  |
  = note: `&&std::vec::Vec<&str>` is not an iterator; maybe try calling `.iter()` or a similar method
  = note: required by `std::iter::IntoIterator::into_iter`

[djc]

This is now covered by test_nested_for.