Dhall specifications

[aviaviavi]

This is being worked on in PR #11, but progress has stalled. I've been running into issues that I've copied here:

• It's not clear how to encode arbitrary json as a Dhall type. Since curl runnings specs are lists of test cases, it seems like the matchers all need to be the same type and it's not clear how to achieve this. Users need to be able to specify json of any shape, and we will never know that shape ahead of time. Curl runnings is currently using aeson for json support, but I can't make an Interpret instance of aeson's Value without having an orphan instance in the hackage library (right?).
• The curl runnings spec uses sum types pretty heavily, and encoding those in Dhall is extremely verbose. There's probably a way to specify some Dhall based utilities to ease the boilerplate, but I haven't been able to get anything like that working yet.
  If you feel like you're up for helping by either advising or contributing parts of the implementation, please reach out!```