[explorer] Add a load command

Loads a board position from a FEN string.

Plumb through setting the Zobrist state on an existing board. Rebuild the hash
when setting the state.

board/src/board.rs

@@ -306,6 +306,15 @@ impl Board {
             zobrist.set_hash_value(new_hash);
         }
     }
+
+    pub fn zobrist_state(&self) -> Option<Arc<ZobristState>> {
+        self.zobrist_hash.as_ref().map(ZobristHash::state)
+    }
+
+    pub fn set_zobrist_state(&mut self, state: Arc<ZobristState>) {
+        self.zobrist_hash = Some(ZobristHash::new(state));
+        self.recompute_zobrist_hash();
+    }
 }
 
 impl Board {