Refactor using Clippy nursery and unwrap_used

src/fen.rs

@@ -123,9 +123,9 @@ fn en_passant_target_square(square: &str) -> Result<u8, FenError> {
 
     match sqr.next() {
         Some(file) if files.contains_key(&file) => match sqr.next() {
-            Some(rank) if ranks.contains_key(&rank) => {
-                Ok(ranks.get(&rank).unwrap() * 8 + files.get(&file).unwrap())
-            }
+            Some(rank) if ranks.contains_key(&rank) => Ok(ranks.get(&rank).expect("Invalid rank")
+                * 8
+                + files.get(&file).expect("Invalid file")),
             Some(_) | None => Err(FenError::from(
                 "Not a valid rank (3 or 6) for an en passant target square",
             )),