elo: multithreading improvements

src/agent.rs

@@ -4,7 +4,8 @@ use std::io;
 use std::io::prelude::*;
 
 #[allow(dead_code)]
-pub trait Agent {
+// implements `Send` so we can use it in a multithreaded `EloArena`
+pub trait Agent: Send {
     /// Returns the move of an [`Agent`]
     fn next_move(&mut self, board: &Board) -> Option<CoordPair>;
     /// Returns the name of the [`Agent`]