[bitboard] Implement BitBoard::is_single_square()

Returns true if there's only one square set in the bitboard.

bitboard/src/bitboard.rs

@@ -78,6 +78,10 @@ impl BitBoard {
         let sq_bb: BitBoard = sq.into();
         *self &= !sq_bb
     }
+
+    pub fn is_single_square(&self) -> bool {
+        self.0.is_power_of_two()
+    }
 }
 
 impl BitBoard {