Loop index out of bounds

[dtolnay]

Using the fix from #8, something is wrong with the loop index tracking.

{% for inner in nested %}
  outer = {{ loop.index0 }}
  {% for _ in inner %}
    inner = {{ loop.index0 }}
  {% endfor %}
{% endfor %}

thread 'main' panicked at 'index out of bounds: the len is 1 but the index is 1', /checkout/src/libcollections/vec.rs:1410

From a quick read, it looks like the _loop_cur variable is never decremented.

[djc]

This is now covered by test_nested_for.