graph/...: use int64 IDs
Created by: kortschak
Two commits that deserve careful review are "graph/topo: update for int64 IDs" (the Bron-Kerbosch routine) and "graph/community: update for int64 IDs" which is a little subtle.
@vladimir-ch @mewmew Please take a look.
After #68.
Updates #31. Fixes #69 (closed). Updates gonum/graph#197. Closes gonum/graph#114.