Avalanche snowman consensus

Rust implementation of AvalancheGo/snow/consensus (including tests).

lib.rs:

avalanche-consensus

avalanche-consensus includes the data structures and algorithms necessary to execute the novel Avalanche consensus engine.

Support is included for Snowball consensus, Slush, and blocks. See https://docs.avax.network/learn/avalanche/avalanche-consensus and the Avalanche whitepaper for more information.