moved map_option to NetCore_Util; deleted unused code

Int32Ord is not necessary. You can do Map.Make (Int32), Set.Make (Int32)
Also, if we need to define NateFosterMap, we should just use core, IMO.
NetCore_Digraph appears to be unused by FatTire and Pathetic

I see that this code is in Merlin too, where I think it should be.
oasis setup
Merge pull request #114 from arjunguha/master

Deleting code