Speed up as_adj_list if igraph_opt("return.vs.es") is false

[cfhammill]

Also updates the docs to point out that this option exists. Not a fully satisfying solution to #194, but provides a simple work around.

[gaborcsardi]

Thanks! I assume that you checked that after addind create_vs, it is still fast. :)

[cfhammill]

It is fast if you set igraph_options(return.vs.es = FALSE). I don't know if create_vs could be sped up. It looks like the first major time waster is V(graph), this takes 9ms, so for a 40000 node graph, it should take 360 odd seconds. After that, adding the graph reference 40000 times takes about 6 seconds. So if create_vs could take a vertex list corresponding to the graph or cache the vertex list it would be a pretty substantial speed up.

[vtraag]

@cfhammill , I guess that PR #197 renders this PR obsolete and should be preferred over this one?

[ntamas]

This PR seems to be superseded by #197 so I'm closing this.