Add as_ref builtin

[GuillaumeGomez]

Fixes #330.

Following discussions with @djc, to keep askama template syntax as simple as possible and to go around the limitations described in #330, a good way to fix it is to add a as_ref builtin filter.

[djc]

Seems kinda reasonable to me. @Kijewski @vallentin any thoughts?

[Kijewski]

The PR is well implemented, and (except for the small clippy warning), LGTM.

But another alternative would be to add prefix operators & and * to the language. This could also be a fix for e.g. #894. Both options would work for me, this PR or adding &+*, and I have no strong preferences.

[djc]

But another alternative would be to add prefix operators & and * to the language. This could also be a fix for e.g. #894. Both options would work for me, this PR or adding &+*, and I have no strong preferences.

I'm not a fan of that particular alternative (https://github.com/djc/askama/pull/937#issuecomment-1874648650 motivated this PR). I don't have much of an objection to this approach, although I don't love it.

[GuillaumeGomez]

Fixed the clippy lint. :)

[GuillaumeGomez]

Updated sentence.

[djc]

There's still/again a clippy failure? (I approved this already, so feel free to go ahead and merge once it passes CI.)