Fix graph_rewire "can't rewire" bug
This finally fixes in the bug addressed by commit 309ddbbd, where parallel edges could be erroneously created. In fact, the bug was more serious: The source and target edge lists always pointed to the same list (in the uncorrelated case, but could occasionally happen for the correlated case also) which got shuffled during iteration. Since the shuffling of one list interfered with the shuffling of the other, some combinations of source and target edges could simply never be considered... This commit forces both lists to always be independent.