[board] Implement a TrailingBitScanner

This bit scanner iterates populated bits in a BitBoard from the trailing (least-significant) end.

Rename BitScanner → LeadingBitScanner.
Factor implementation of these two into a macro.
Clean up iterator returned by BitBoard::occupied_squares
Implement BitBoard::occupied_squares_trailing

board/src/bitboard/bitboard.rs

@@ -1,8 +1,8 @@
 // Eryn Wells <eryn@erynwells.me>
 
 use super::library::{library, FILES, RANKS};
-use super::BitScanner;
-use crate::Square;
+use super::{LeadingBitScanner, TrailingBitScanner};
+use crate::{square::Direction, Square};
 use std::fmt;
 use std::ops::{BitAnd, BitAndAssign, BitOr, BitOrAssign, Not};
 
@@ -62,11 +62,16 @@ impl BitBoard {
 }
 
 impl BitBoard {
+    /// Return an Iterator over the occupied squares, starting from the leading
+    /// (most-significant bit) end of the field.
     pub(crate) fn occupied_squares(&self) -> impl Iterator<Item = Square> {
-        BitScanner::new(self.0)
-            .map(|x| u8::try_from(x))
-            .take_while(|x| x.is_ok())
-            .map(|x| Square::from_index(x.unwrap() as usize))
+        LeadingBitScanner::new(self.0).map(Square::from_index)
+    }
+
+    /// Return an Iterator over the occupied squares, starting from the trailing
+    /// (least-significant bit) end of the field.
+    pub(crate) fn occupied_squares_trailing(&self) -> impl Iterator<Item = Square> {
+        LeadingBitScanner::new(self.0).map(Square::from_index)
     }
 }