[core] Implement Square::file_rank()

Returns a tuple of the square's file and rank.

core/src/coordinates.rs

@@ -208,6 +208,10 @@ impl Square {
         unsafe { Rank::new_unchecked((self as u8) >> 3) }
     }
 
+    pub fn file_rank(&self) -> (File, Rank) {
+        (self.file(), self.rank())
+    }
+
     pub fn neighbor(self, direction: Direction) -> Option<Square> {
         let index: u8 = self as u8;
         let dir: i8 = direction.to_offset();