[position] Export Position::zobrist_hash()

Pass-through call to self.board.zobrist_hash() to return the hash value of the
current board position.

position/src/position.rs

@@ -197,6 +197,13 @@ impl Position {
     }
 }
 
+impl Position {
+    #[must_use]
+    pub fn zobrist_hash(&self) -> Option<u64> {
+        self.board.zobrist_hash()
+    }
+}
+
 impl Position {
     pub fn display(&self) -> DiagramFormatter {
         self.board.display()