Clean up chapter 2 extern crates that aren't needed, and add a lot more help with learning Rust while you learn Roguelikes.

chapter-02-helloecs/src/main.rs

@@ -1,6 +1,4 @@
-extern crate rltk;
 use rltk::{Console, GameState, Rltk, RGB, VirtualKeyCode};
-extern crate specs;
 use specs::prelude::*;
 #[macro_use]
 extern crate specs_derive;