Convenience function to find the strongly connected component of a particular object

Often I compute the collection of strongly connected components of a snapshot just to extract the SCC of a particular object:

```
snapshot = refcycle.snapshot()
scc = next(scc for scc in snapshot.strongly_connected_components() if object_of_interest in scc)
```

It would be great to be able to write `scc = snapshot.scc_containing(object_of_interest)` instead.