Release Paxos version 19.0.0-alpha.18.
Release Paxos version 19.0.0-alpha.18.

 * Merge `master`.
 * Upgrade `avenue` to `0.6.18`.
 * Upgrade `whittle` to `0.0.6`.
 * Update `LICENSE` for current year.
 * Sketch of Rust shaper.
 * Start of Rust implementation.

Closes #479.

bigeasy/paxos,bigeasy/paxos