Added a simple benchmark.

Given the goal of finding all paths of four edges in a graph, the benchmark
first uses comprehend. Next, it tries a naive core.logic approach, where a
vector is used as the 'database'.

On my current machine I get the following results:
- The naive approach is faster initially
- At around 40 total edges comprehend overtakes the naive approach
- At 150 total edges the naive approach takes nearly 5 times longer than comprehend
Added information on how long it takes to construct the graph versus constructin
g the indexed set with the same data.