[bitboard] Replace separate methods for leading and trailing iteration

Add chessfriend_bitboard::IterationDirection
Make BitBoard::occupied_squares() take an IterationDirection and return an iterator
corresponding to the direction.
Do the same for ::first_occupied_square().

bitboard/src/direction.rs

@@ -0,0 +1,6 @@
+#[derive(Default)]
+pub enum IterationDirection {
+    #[default]
+    Leading,
+    Trailing,
+}