Update to newest dhall (& hnix)

[Profpatsch]

There have been a few changes with dhall (multiple let bindings, Some/None and the new Map module).

hnix got a new major version release a few days ago, but for now I’ll update to the older one.

Please assign me to this issue.

[Gabriella439]

I'm also going to merge this repository into the dhall-haskell repository now that this builds against the latest version of dhall