[board] Implement a BitScanner on u64

This struct implements an Iterator that returns the positions of the 1 bits in the u64.
BitBoard takes this iterator and maps it into Squares in the pieces() method.

This is laying the groundwork for an iterator over the pieces in a Position. 👀

board/src/bitboard/bit_scanner.rs

@@ -0,0 +1,67 @@
+// Eryn Wells <eryn@erynwells.me>
+
+use super::BitBoard;
+use crate::square::Square;
+
+pub(crate) struct BitScanner {
+    bits: u64,
+    shift: u32,
+}
+
+impl BitScanner {
+    pub(crate) fn new(bits: u64) -> BitScanner {
+        BitScanner { bits, shift: 0 }
+    }
+}
+
+impl Iterator for BitScanner {
+    type Item = u32;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        if self.shift == u64::BITS {
+            return None;
+        }
+
+        let shifted_bits = self.bits << self.shift;
+        let leading_zeros = shifted_bits.leading_zeros();
+
+        if leading_zeros == u64::BITS {
+            self.shift = leading_zeros;
+            return None;
+        }
+
+        let position = u64::BITS - (self.shift + leading_zeros + 1);
+        // Shift 1 additional place to account for the 1 that `leading_zeros` found.
+        self.shift += leading_zeros + 1;
+
+        Some(position)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn zero() {
+        let mut scanner = BitScanner::new(0);
+        assert_eq!(scanner.next(), None);
+    }
+
+    #[test]
+    fn one() {
+        let mut scanner = BitScanner::new(1);
+        assert_eq!(scanner.next(), Some(0));
+        assert_eq!(scanner.next(), None);
+    }
+
+    #[test]
+    fn complex() {
+        let mut scanner = BitScanner::new(0b11000101);
+        assert_eq!(scanner.next(), Some(7));
+        assert_eq!(scanner.next(), Some(6));
+        assert_eq!(scanner.next(), Some(2));
+        assert_eq!(scanner.next(), Some(0));
+        assert_eq!(scanner.next(), None);
+    }
+}