gh-37571: Improve method `_ford fulkerson chronicle` in `src/sage/combinat/posets/posets.py`
We relabel the internal digraph to integers in range `0..n-1` and
associate each edge an index in range `0..m-1` to use less dictionaries.
We also do some minor changes in `posets.py`.
### :memo: Checklist
<!-- Put an `x` in all the boxes that apply. -->
- [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.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation accordingly.
### :hourglass: Dependencies
<!-- List all open PRs that this PR logically depends on. For example,
-->
<!-- - #12345: short description why this is a dependency -->
<!-- - #34567: ... -->
URL: https://github.com/sagemath/sage/pull/37571
Reported by: David Coudert
Reviewer(s): Frédéric Chapoton
