[WIP] Graph types and graph convnet demo

[duvenaud]

There are three common representations of graphs:

• Adjacency matrices n=>n=>Bool: An n by n Boolean matrix where True means an edge.
• Edge lists List (n & n): A list of pairs (n & n) meaning an edge from left to right.
• Adjacency or child lists n=>List n: A table of lists of each node's children.

This demo has functions for all 6 combinations of casting from one
representation to another. All can in principle be paralellized by the compiler due to the use of the
accum monoid.

I also implemented a basic graph convnet.