now precomputers the shortest path between all pairs of switches and uses
this to computer paths through the switch fabric.  In the process of
writing this I also removed the limitations of two switch hops.  Packets
can now travel through any number of switches to reach their destination.
Of course, the longer the path the more it costs so assign will prefer
shorter paths.

Also did various other tweaks in the process.  We now use strings almost
everywhere instead of char*'s and the makefile is cleaner.