[position] Move unmake move stuff to an unmake_move module

position/src/position.rs

@@ -3,6 +3,7 @@
 mod captures;
 mod make_move;
 mod position;
+mod unmake_move;
 
 pub use {
     make_move::{MakeMoveError, ValidateMove},

position/src/position/make_move.rs

@@ -60,8 +60,6 @@ pub enum MakeMoveError {
     PromotionRequired(Square),
 }
 
-pub enum UnmakeMoveError {}
-
 impl Position {
     /// Make a move in the position.
     ///

position/src/position/unmake_move.rs

@@ -0,0 +1,3 @@
+// Eryn Wells <eryn@erynwells.me>
+
+pub enum UnmakeMoveError {}