[board, core, bitboard] Clean up casts between Rank, File and BitBoard

Let BitBoard::rank and BitBoard::file take a Rank and File directly, instead of a
u8 by reference. And then make the Rank/File::as_index const and return a value
rather than a reference.

All this allows you to convert between Rank, File, and BitBoard at compile tile
(i.e. as a const method) rather than needing to do runtime stuff.

core/src/coordinates.rs

@@ -88,8 +88,8 @@ macro_rules! range_bound_struct {
             }
 
             #[must_use]
-            $vis fn as_index(&self) -> &$repr {
-                &self.0
+            $vis const fn as_index(&self) -> usize {
+                self.0 as usize
             }
 
             $vis fn iter(&self) -> impl Iterator<Item = Self> {