dhall-to-{json,yaml,yaml-ng}: support for recursive types

[WillSewell]

Support recursive types as described in https://docs.dhall-lang.org/howtos/How-to-translate-recursive-code-to-Dhall.html.

dhall-to-{json,yaml,yaml-ng} already has special case support for the recursive JSON type:

https://github.com/dhall-lang/dhall-haskell/blob/4d3bb4784908f4ddb63ee70946a84fa967ca9733/dhall-json/src/Dhall/JSON.hs#L503-L589

In https://github.com/dhall-lang/dhall-kubernetes/issues/111#issuecomment-583983148, @Gabriel439 suggested adding support for arbitrary recursive types. I thought it was better to create a separate issue to track this because the limitation is more general than that particular instance of the issue.

I would like to help with this, but to be honest I'm not sure how successful I would be given my current Dhall experience (understanding how to model recursive types was tricky enough!). However if anyone could give any hints/pointers I could give it a go.