feat(Combinatorics/SimpleGraph): vertices are not reachable iff set of walks between them is empty

[Rida-Hamadani]

This lemma came up while extending SimpleGraph.dist to ENat.

Co-authored-by: Kyle Miller [email protected]

---

[github-actions[bot]]

PR summary 4a359bb1bb

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

---

Declarations diff

+ not_reachable_iff_isEmpty_walk

You can run this locally as follows

## summary with just the declaration names:
./scripts/no_lost_declarations.sh short <optional_commit>

## more verbose report:
./scripts/no_lost_declarations.sh <optional_commit>

[IvanRenison]

👍

[b-mehta]

bors merge

[mathlib-bors[bot]]

Pull request successfully merged into master.

Build succeeded:

• Build
• Lint style