Introduce a Loop object

[fabpot]

Fixes #4096

[stof]

Suggested change

	available and a ``RuntimeException`` is thrown if you retry to use them.
	available and a ``RuntimeException`` is thrown if you try to use them.

[stof]

I just realized that by using a Loop object, assigning it to a different variable (like outer_loop will now work fine, making it much easier to reference the loop context of an outer loop when using nested loops (especially when the number of nested loops can change) because it removes the need for loop.parent.loop.parent.loop:

{% for item in items %}
    {% set outer_loop = loop %}
    {% for property in item.properties %}
        {% for values in property.values %}
            {{ outer_loop.index0}} {{property.name}} {{loop.index0}}: {{value}}
        {% endfor %}
    {% endfor %}
{% endfor %}

And this makes it more similar to the way loops work in Jinja (this code snippet would also be valid Jinja code, while loop.parent is not)