Add bench profiles to Cargo.toml to force codegen-units=1

Add bench profiles to Cargo.toml to force codegen-units=1

This makes a small difference for NetworkGraph deserialization
as it enables more inlining across different files, hopefully
better matching user performance as well.

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

test routing::network_graph::benches::read_network_graph  ... bench: 2,037,875,071 ns/iter (+/- 760,370)
test routing::network_graph::benches::write_network_graph ... bench: 320,561,557 ns/iter (+/- 176,343)