Implement iterator for neighbors to avoid static_cast call

```
Neighbor iteration should be encapsulated so the user does not have to call
static_cast every time.
```

Original issue reported on code.google.com by `bombax` on 10 Feb 2012 at 6:10

The issue names no files, tests, or entry points. Start by locating the neighbor traversal code and the static_cast calls users currently need, then determine the intended iterator interface; done means callers can iterate neighbors without performing those casts, with relevant tests updated or added if they exist.

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