Add Rust-based route construction to wasm client

[futurechimp]

The current was client demo retrieves a localnet topology, constructs a route in pure JS, and builds a Sphinx packet from that. However route construction wouldn't stand up to real-world usage (have a look at it).

Let's have the wasm package depend on our "normal" platform route generation code in Rust. So we want a Rust function that can take topology JSON (a string) and return a correct Route.