gh-38674: cythonize the chain polynomials of posets
for the sake of speed
### :memo: Checklist
- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [x] I have created tests covering the changes.
- [x] I have updated the documentation and checked the documentation
preview.
URL: https://github.com/sagemath/sage/pull/38674
Reported by: Frédéric Chapoton
Reviewer(s): David Coudert, Frédéric Chapoton
