EdgeIterables#getEdges() should reuse Edge objects

Since some of our vertex implementations don't allow for mutating or keeping references to edges returned by Vertex#getEdges(), we can consider that part of the API and exploit it in other places.
EdgeIterables#getEdges() currently creates a new Edge object at each iteration.
We can reuse the object for efficiency.

Note: this issue may become irrelevant depending on how we refactor the Vertex API (e.g. no distinction between Vertex and SimpleVertex).