Sync Forest and its helpers with recent changes in libsemigroups

[james-d-mitchell]

Will require

libsemigroups/libsemigroups#847

to be merged before the ci here will pass.

[Joseph-Edwards]

This looks good to me. I've requested one small doc change, and made one comment, but otherwise I'm happy for this to be merged.

It's worth noting that we expect this to fail the conda libsemigroups jobs. This raises the question that maybe they should only be run when making PRs into stable-*, or on a release? What do you think @james-d-mitchell?

[james-d-mitchell]

This looks good to me. I've requested one small doc change, and made one comment, but otherwise I'm happy for this to be merged.

It's worth noting that we expect this to fail the conda libsemigroups jobs. This raises the question that maybe they should only be run when making PRs into stable-*, or on a release? What do you think @james-d-mitchell?

I think that'd be a good idea, this way we'll only see proper errors, not out of sync issues like this.