mathlib3

e9b1fbd0 - feat(combinatorics/simple_graph): add maps and subgraphs (#8223)

feat(combinatorics/simple_graph): add maps and subgraphs (#8223) Add graph homomorphisms, isomorphisms, and embeddings. Define subgraphs and supporting lemmas and definitions. Also renamed `edge_symm` to `adj_comm`. Co-authored-by: Kyle Miller <kmill31415@gmail.com>