Allow to create mutable template variables

[GuillaumeGomez]

In case you have iterators, like:

{% let fields = self.fields_iter() %}
{% if fields.peek().is_some() %}
    {% for (field, ty) in fields %}
    {% endfor %}
{% endif %}

It would make it very useful.

[Kijewski]

This would break documented semantics quite severely, because to make this this useful, {% let x = y %} should re-assign x instead of shadowing it.

Maybe we could add a mut keyword for Target?

[GuillaumeGomez]

I like the idea of adding a mut keyword. 👍

[GuillaumeGomez]

Done in #462 and #710. Closing then!