Add {% break %} and {% continue %}

[Kijewski]

This PR adds {% break %} and {% continue %} statements to break out
of a loop, or continue with the next element of the iterator.

[djc]

This looks like a nice straightforward implementation, but it looks like this would not throw any error if {% break %} or {% continue %} are added outside of a loop context -- I think it probably should. Any ideas? (I think it might be easier to do it at the codegen stage than in the parser.)

[Kijewski]

I implemented the check inside of the parsing step. It was easier to implement, and it has the added bonus that the user gets informed about the exact position of the offending {%break%}.

This was the first time I had to use a Cell. Does this mean I can call myself a Rust programmer now? ;)

[djc]

Why do we need to use Cell here? Could we just pass &mut State<'a> around here? (Also I think we could pass State<'_> rather than State<'a>, would you mind checking that?)

(Here, too, I wonder: do you have use cases for these constructs?)

[djc]

Note that Jinja does not allow this:

Unlike in Python, it’s not possible to break or continue in a loop.

[Kijewski]

Replaced State<'a> with State<'_>. I didn't think much about it because all the functions already used lifetime argument, and replaced Syntax<'a> did so, too.

State cannot be passed mutable because it's captured by multiple closures.

Here, too, I wonder: do you have use cases for these constructs?

I used it before in Flask apps. You don't have to use break or continue in templates, since you can always put more logic into the Rust code and less in the template code, but I guess that's a matter of taste.

Note that Jinja does not allow this:

Unlike in Python, it’s not possible to break or continue in a loop.

After 13 years, can we call an extension part of the standard? :)

[djc]

If you rebase this, I'm happy to merge it. Sorry for the long review cycle!

[Kijewski]

No problem. Rebased.

[djc]

Thanks!