`dhall start-refactor` proposal

[german1608]

I was doing a refactor on the Prelude (https://github.com/dhall-lang/dhall-lang/pull/1026) and I see that it could be useful to have a dhall start-refactor command that:

1. When execute it on a --file, it will listen to all the files related on the imports (if any).
2. When a file is saved, the hash will be recomputed and shown at screen, showing if there is a difference with the initial hash before doing any changes.

There is the watch unix command but it doesn't say if there was a difference through the time of editing.

[Gabriella439]

@german1608: Yeah, I think this would be useful! In addition to displaying the hash, it might also be worth recording the old expression in memory and comparing against the new expression using Dhall.Diff