Add Pijul resolver

[notramo]

Pijul is a distributed version control system. It's inspired by Darcs, but is written in Rust, and uses a different merge algorithm.
https://pijul.org/

[notramo]

@straight-shoota I am interested in implementing this. Would you accept a pull request?

[straight-shoota]

I'm not very eager on this one. Introducing new resolver isn't a huge effort for the maintainers, if you're contributing the code. We'd still need to review and approve it. However, continuously maintaining the resolver is a more demanding task. I'm hesitant to do that for a niche feature.

Honestly, I've never heard about Pijul before. We've recently added some alternative SCM resolvers besides git, but they seem to be a lot more popular than Pijul. And even that was a big leap, considering how much baggage we might now be dragging along. I would rather wait a bit to see how things go with the latest additions, before incorporating new, even lesser used resolvers.

For the meantime, maybe you could fork shards and add a Pijul resolver in your fork. This would allow people to use it, without immediately adding a maintenance burden on the upstream project.
And please direct interested parties to this issue to give it a thumbs up, so we can gauge demand for this resolver.