FEAT: Speed up Graph, StableGraph::map

Use Vec::extend instead of a push loop -- this improves performance.

Microbenchmark results:

```
 name              old ns/iter  new ns/iter  diff ns/iter   diff %
 graph_map         569          323                  -246  -43.23%
 stable_graph_map  594          405                  -189  -31.82%
```
2 files changed