gh-36225: improve the use of graphs in `sage/combinat/posets/*`
The main changes are:
- avoid sorting vertices or edges when not required
- avoid asking for edge labels when not needed
- use more iterators
- avoid computing all paths from `s` in `_ford_fulkerson_chronicle` (in
`posets.py`)
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URL: https://github.com/sagemath/sage/pull/36225
Reported by: David Coudert
Reviewer(s): Frédéric Chapoton
