Restructure the rewiring code, introducing further abstraction through
class inheritance.

Both uncorrelated and correlated cases draw edges directly.
This has actually proven faster than drawing vertices for the correlated
case, since realizing that indexes could be stored instead of edges.
Doing so avoids changes in the pool of candidate edges, which in turn
removes the need to rebuild it for each edge to rewire.
Consequently, it also makes the uncorrelated case a lot quicker.

In the uncorrelated undirected case, the new code also fixes a serious
bug: when building the edge pool, only one end of each edge was looked
at, because the "edges" vector is not equivalent to drawing all
out_edges from all vertices, as is done now.