PR checklist after 2023-10-12 sync with upstream libsecp256k1

[jonasnick]

After the most recent PR to sync commits from upstream libsecp256k1, some updates to open PRs may be necessary:

• Replace fe_equal_var with fe_equal
• There is now a CHECK_ILLEGAL macro for the API tests which should be used instead of setting the counting callback for a context and then CHECKing that ecount is some number.
• We switched from cirrus CI to github actions which means that any changes to cirrus.yml need to be reproduced in .github/workflows/ci.yml

See also #266 for the previous update.