Add actual implementation on interfaces from TinkerPop3 structure package.

Based on changes from S2GRAPH-129, S2GRAPH-130, it's easy to implement tinkerpop3's structure interfaces directly.

This issue is about change our previous core entity Edge/Vertex/Graph into S2Edge/S2Vertex/S2Graph.

I suggest to focus on naive implementation first on this issue, then improve tp3 integration while we discuss.

Of course test cases, documentations should be followed up.

I suggest to create more subtask under S2GRAPH-72, so we can track them easily.