solvable part of CohenCyril's abel backport

[Tragicus]

Motivation for this change

Extracts the part from https://github.com/math-comp/math-comp/pull/944 that concern solvable (with a few things moved to zmodp).

Depends on:

• https://github.com/math-comp/math-comp/pull/1196

Things done/to do

• [x] added corresponding entries in CHANGELOG_UNRELEASED.md

• [ ] added corresponding documentation in the headers
• [x] tried to abide by the contribution guide

Compatibility with MathComp 1.X

• [ ] I added the label TODO: MC-1 port to make sure someone ports this PR to the mathcomp-1 branch or I already opened an issue or PR (please cross reference).

Automatic note to reviewers

Read this Checklist and put a milestone if possible.

[thery]

I have just starting reviewing before realizing that actually the code that I was reviewing was already merged :disappointed: Can someone that knows this PR resolve the conflicts. Thanks

[Tragicus]

I am quite confused, I got rid of the commits for the previous PRs and rebased on top of master, it should be correct now.

[Tragicus]

Sad, I hoped I could avoid compiling myself...

[thery]

I don't understand why the CI is happy. Me I get

File "./algebra/zmodp.v", line 396, characters 11-18:
Error: The variable tpermJt was not found in the current environment.

[Tragicus]

The CI does too, does it not ?

[thery]

some do, the majority succeeds

[Tragicus]

Yes, but a lot of them do not need algebra. The ones who do were skipped (as far as I understand).

[CohenCyril]

@Tragicus is it ready for review again (can we undraft?)

[Tragicus]

Is there something left to do here, beyond rebasing?

[thery]

@Tragicus if you say you are done I can have a look again

[Tragicus]

I think so. As an orthogonal matter of fact, I just noticed that we should be able to remove a few more _.+1 once https://github.com/math-comp/math-comp/pull/1256 is merged.

[thery]

@Tragicus can you resolve the conflicts?

[thery]

fine with me. @CohenCyril you can merge.