Create the fork-choice crate.

- Adds the naive fork choice (longest chain) rule.
- Adds basic documentation for the crate.

eth2/fork_choice/Cargo.toml

@@ -0,0 +1,11 @@
+[package]
+name = "fork_choice"
+version = "0.1.0"
+authors = ["Age Manning <Age@AgeManning.com>"]
+edition = "2018"
+
+[dependencies]
+db = { path = "../../beacon_node/db" }
+ssz = { path = "../utils/ssz" }
+types = { path = "../types" }
+