Validate Safety of minijinja-stack-ref

[mitsuhiko]

Now that #148 landed the practical limitations (other than excessive uses of Arc all the way down) are really lifetimes. I tried to use the functionality and the main way to find this enjoyable really requires borrowing from data that is already there (eg: on the stack).

I created an API around this which hopefully provides a safe interface to do so in minijinja-stack-ref but it's unclear if this is safe. So I need to check the create for that.

Motivating example

[mitsuhiko]

cc @SergioBenitez would be curious to hear if this is useful to you btw.

[mitsuhiko]

Likely the Deref has to go because it lets one hold on to a ref too long. For the use within minijinja that wouldn’t be necessary.

[mitsuhiko]

This should be safe now by using a with based API internally.