Learn to write fast Dag == methods

This release adds:

1. a depth method on DAG to compute the max depth from a source node a given node is in the graph.
2. RefPair: a tuple2 that uses reference equality for equality.
3. a Non-exponential (in fact linear) memoized equality for Literal[N, T] which unfortunately should be copied on your Dags N[T]. If you don't have an efficient equality, and the naive case class equality can be exponentially expensive for Dags that have fan-in, then running rules can be exponentially slow.