Add a trivial benchmark of calculating routes on today's graph

Add a trivial benchmark of calculating routes on today's graph

Sadly rust upstream never really figured out the benchmark story,
and it looks like the API we use here may not be long for this
world. Luckily, we can switch to criterion with largely the same
API if that happens before upstream finishes ongoing work with the
custom test framework stuff.

Sadly, it requires fetching the current network graph, which I did
using Val's route-testing script written to test the MPP router.