Downstreams for renaming

[voodoos]

1. Distinguish uids from interfaces and implementations: [tooling] Distinct uids for interfaces ocaml/ocaml#13286

There should be a cleaner way to store the additional information. Upstream this info is part of a Unit_info.t record that is stored as the "current unit" in the environement. But here in flambda-backend it is Compilation_unit.t instead. This looks like conflicting refactorings that it would be useful to reconcile.
Here, to keep the prototype simple, we use a tuple to add that information.

2. Linked related declarations together: [tooling] Remember linked declarations ocaml/ocaml#13308

[voodoos]

@lukemaurer what do you think would be the best way to handle the additionnal information in bcfebba ?

In the upstream compiler, we now store a Unit_info.t as the "current unit", but in flambda backend it is a Compilation_unit.t which is less obviously extendable. Here, to keep the prototype simple, I used a tuple to add the intf/impl information.

[mshinwell]

@lukemaurer I'd like to discuss the Unit_info part of this

[lukemaurer]

Hmm. I think it would make more sense to do what we've been intending to do for a while and have Unit_info.t store a Compilation_unit.t rather than a modname, and then Env.{get,set}_unit_name can just do exactly what they do upstream. The only reason we didn't do that before is because it didn't make sense in the middle of a big merge.

(Ultimately I'm not sure that Compilation_unit.t is the right type here as opposed to Compilation_unit.Name.t, but that can be another PR.)