[position] Move castle evaluation code to its own submodule of position

Move the tests too.

position/src/position.rs

@@ -1,9 +1,11 @@
 // Eryn Wells <eryn@erynwells.me>
 
+mod castle;
 mod make_move;
 mod position;
 
 pub use {
+    castle::CastleEvaluationError,
     make_move::{MakeMoveError, ValidateMove},
-    position::{CastleEvaluationError, Position},
+    position::Position,
 };