[bitboard] Implement BitBoard::population_count()

Uses u64::count_ones() to return the population count of the BitBoard.

bitboard/src/bitboard.rs

@@ -80,6 +80,20 @@ impl BitBoard {
         !(self & square_bitboard).is_empty()
     }
 
+    /// The number of 1 bits in the BitBoard.
+    ///
+    /// ## Examples
+    ///
+    /// ```
+    /// use chessfriend_bitboard::BitBoard;
+    /// assert_eq!(BitBoard::EMPTY.population_count(), 0);
+    /// assert_eq!(BitBoard::new(0b01011110010).population_count(), 6);
+    /// assert_eq!(BitBoard::FULL.population_count(), 64);
+    /// ```
+    pub fn population_count(&self) -> u32 {
+        self.0.count_ones()
+    }
+
     pub fn set_square(&mut self, sq: Square) {
         let sq_bb: BitBoard = sq.into();
         *self |= sq_bb