Add benchmark of deserializing a NetworkGraph.

Add benchmark of deserializing a NetworkGraph.

NetworkGraph is one of the largest structures we generally
deserialize, so it makes for a good benchmark, even if it isn't the
most complicated one.

As of this commit, on an Intel 2687W v3, these benchmarks take:

test routing::network_graph::benches::read_network_graph  ... bench: 2,101,420,078 ns/iter (+/- 6,649,020)
test routing::network_graph::benches::write_network_graph ... bench: 344,696,835 ns/iter (+/- 229,061)