Adapt to https://github.com/math-comp/math-comp/pull/1031

[proux01]

No description provided.

[CohenCyril]

I'm afraid this conflicts a bit with #80 which performs a huge cleanup, and which I was about to review and probably merge.
@pi8027 @proux01 how would you prefer to handle this?

[proux01]

@CohenCyril note that this actually is a single char change: 48aef72

The actual question is the interaction between #80 and #42
If #80 is merged rapidly, I'm fine rebasing #42 on top of it, otheriwse I really think we shouldn't wait too much to merge the HB port now that MC 2 is out (not having multinomials means not having CoqEAL which makes MC2 not yet usable for many users).
Maybe the best solution is to merge #42 now and rebase #80 (I can help if needed but this should be relatively easy from what I can see, it doesn't seem to really touch the structures).