Solution to countdown problem in Rust

This is a solution to the countdown problem presented in this paper in Rust.

How to run

$ cargo run

Benchmark

Benchmarks with the criterion library.

# Serial Version
$ cargo bench
     Running target/release/deps/count_down_bench-05c9b5e9abb05e85
CountDown               time:   [368.36 ms 370.71 ms 373.22 ms]

# Parallel version
$ cargo bench
     Running target/release/deps/count_down_bench-3a06313c685b4b6b
CountDown               time:   [69.206 ms 71.958 ms 74.038 ms]