Reduced index size.

- Index vertex does not store the entire id of its child vertices
anymore, but only the delta.
- Optimizing the edge representation after graph loading by
transforming them from a list to an array.
- Removed triple vertices and replaced look-ups with binary search on
the third index level, now called BindingIndexVertex.
- Using CompactIntSet to further reduce the index sizes on levels 1 and
2.