`Graph.getAdjacentEdges` had a serious defect: for the adjacent edges,
it's hard to tell which of the {src,dst} is the neighboring node address
and which is the node we called `getAdjacentEdges` on.
This commit fixes that limitation by replacing `getAdjacentEdges` with
`getNeighborhood`, with a return signature of
`{edge: Edge<EP>, neighborAddress: Address}[]`
Some yak shaving was required: we needed a version of `expectSameSorted`
and, by extension, `sortedByAddress` that takes an accessor to an
Addressable, so that we could test that the neighborhoods returned were
correct. To satisfy flow, I created `expectSameSortedAccessorized` and
`sortedByAddressAccessorized`. Cumbersome, but it worked. ¯\_(ツ)_/¯
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