Support crypton

[endgame]

cryptonite has been forked to crypton for ongoing maintenance. We should support it with a flag, defaulting to crypton and falling back to cryptonite.

[mbj]

Why the flag? Context: I'd be happy to send a PR to do the change but would be interested to know why we'd want the flag?

[endgame]

That was an initial guess, before I saw which way things were going to go in the community. I haven't had time to think much about this; are people writing their packages to support both cryptonite and crypton, or has everyone just moved over to crypton?

[mbj]

So far I've seen just moves to crypton and issues that just ask to move to crypton.

• Migrate to crypton snoyberg/conduit#506
• using crypton yesodweb/wai#931
• Avoid crypton dependency for now haskell-servant/servant#1686 (this one stayed on cryptonite for the moment)
• Move to crypton, update CI snoyberg/http-client#506

none of these have so far AFAIK used a flag. (there are some more PRs unmerged as of yet, also not doing flags).

[endgame]

If those projects beneath us in the depgraph don't use a flag then there's no point us doing so either. The PR should be pretty straightforward, then. Only subtlety is you might need to change the cabal.ede template and (in a separate PR, targeting your main PR) run scripts/generate --commit to rebuild the service bindings.