json-to-dhall initial implementation

[antislava]

Description: json-to-dhall(s, j) -> d, where j is a valid JSON being converted, s is a Dhall type (schema), and d is the resulting Dhall value (term).

Background: json-to-dhall · Issue #878 · dhall-lang/dhall-haskell

Note: All the conversion logic is currently in Main.hs and will have to be factored out to dhall-json library in the future, to make it usable in other Haskell projects. This should be a separate next step as it will require some coordination with the dhall-to-json logic of the library, following initial practical tests and find-tuning of the json-to-dhall tool, and most likely after some efforts to standardize/formalize JSON <=> Dhall conversion rules (which may also result in the upstreaming of the core logic to dhall-lang).

[Gabriella439]

Excellent work! Just a couple of small comments

I added you as a collaborator so that you can merge whenever you're ready

[albertov]

This is very useful, thanks! Currently using an adapted version while it is in Main