WIP: LG abstraction

[rohitvarkey]

WIP PR Experimenting with using the LightGraphs abstractions.

Constructing and modifying the graph

• Graph
• DiGraph
• add_edge!
• rem_edge!
• add_vertex!, add_vertices!
• rem_vertex!
• zero

Edge/Arc interface

• src
• dst
• reverse
• ==
• Pair / Tuple conversion

Accessing state

• nv
• ne
• vertices (Iterable)
• edges (Iterable)
• neighbors, in_neighbors, out_neighbors
• in_edges
• out_edges
• has_vertex
• has_edge
• has_self_loops (though this might be a trait or an abstract graph type)

Non-Core APIs

• adjacency_matrix
• degree

With respect to #14 , I have an edges iterator implemented in this PR that iterates through all the edges in the Stinger graph. It still needs some checks to skip blank edgeblocks and also needs iterate till finding a valid edgeblock in start. I think I can backport this to replace the current foralledges with a actual Julia iterator after modifying it for the edgeblocks of a vertex.

When this PR gets merged, StingerLG will become Stinger and StingerWrapper will become 0.6 only.