igraph_rewire() should take an igraph_edge_type_sw_t parameter instead of igraph_rewiring_t

[szhorvat]

In igraph 1.0, igraph_rewire() should take an igraph_edge_type_sw_t parameter instead of igraph_rewiring_t. This way we will be able to control separately whether multi-edges and self-loops are allowed to be created.

I plan to implement rewiring procedures for multigraphs. This change prepares the ground for this feature. For now, IGRAPH_UNIMPLEMENTED should be returned for unsupported combinations.

Related: #1062.

[szhorvat]

Ideally it should also return the fraction (or absolute number) of failed rewirings. It's a bit unclear how this will be done when multiple moves are allowed (not just plain edge switches), so perhaps we leave this for a later version.