Add redub to setup-dlang

[MrcSnm]

I've also included some improvements which makes the setup run in parallel when downloading the build tool

[Geod24]

I don't think we should be doing it, it will just fraction the community even more. If you want this to happen, I would try to convince Walter & Atila.

[MrcSnm]

I don't think we should be doing it, it will just fraction the community even more. If you want this to happen, I would try to convince Walter & Atila.

I'm only making this PR since I thought this wasn't an official thing for D. If it indeed is, I have no reason to continue in the PR since I'm not willing to deal with lots of bureaucracy :)

[WebFreak001]

note that you can tell users to use your fork without it needing to be merged as well in case of it being unable to get merged

[the-horo]

I've added #96, if you want to rebase to get a cleaner CI

[Geod24]

I'm only making this PR since I thought this wasn't an official thing for D.

That is a good point. We should move this to dlang