Use PlayerIndex instead of String

src/player.rs

@@ -1,5 +1,14 @@
 use crate::{Gaussian, BETA, GAMMA, N_INF};
 
+#[derive(Copy, Clone, Debug, Default, PartialEq, PartialOrd, Eq, Ord, Hash)]
+pub struct PlayerIndex(usize);
+
+impl PlayerIndex {
+    pub fn new(index: usize) -> Self {
+        Self(index)
+    }
+}
+
 #[derive(Clone, Copy, Debug)]
 pub struct Player {
     pub prior: Gaussian,