Populate the edges of this node if necessary.
The first time this is called it will populate the edges for this node in the graph. It does this by scanning the underlying function, so once this is done, any changes to that function must be explicitly reflected in updates to the graph.
This will not update or re-scan anything if called repeatedly. Instead, the edge sequence is cached and returned immediately on subsequent calls.
the populated EdgeSequence to simplify walking it.