[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/square.rs

@@ -25,7 +25,7 @@ impl Square {
         Ok(Square::from_index_unsafe(index))
     }
 
-    fn from_index_unsafe(index: u8) -> Square {
+    pub(crate) fn from_index_unsafe(index: u8) -> Square {
         Square {
             rank: index / 8,
             file: index % 8,