Change path module to functor; UnitPath replaces old Path
Add Path instance for Network_Common that uses the Int64.t costs on each link as the weight
Change name of Path to NetPath to avoid naming conflict with ocamlgraph
Add mutable weight field to allow reweighting edges for dijkstra's
Add edge_to_string function
path_functor: remove unused module
Merge pull request #46 from frenetic-lang/path_functor

Path functor