score, initialize core functionality and types

Adds Chrono package

Adds a "Result" type to lib.rs

creates helpful type aliases for Score, Boost, and Gravity.

Creates a "TimedScore" which is a value and time data for calculating
score over time. There are two forms of this, Boosted and Decaying.
Since a Score can either be decaying over time or have been boosted
recently and be kept from decaying for some time.

Also implements a BoostedScore and a DecayingScore, which are both
TimedScores which represent the different states. These are important
because you can't boost a BoostedScore for instance.

koucha/src/lib.rs

@@ -0,0 +1,5 @@
+use std::error::Error;
+
+type Result<T> = std::result::Result<T, Box<dyn Error>>;
+
+pub mod score;