Fix some linting errors

day_04/src/crossword.rs

@@ -198,14 +198,14 @@ impl Crossword {
         self.height
     }
 
-    pub fn iter_cells(&self) -> CrosswordIterator {
+    pub fn iter_cells(&self) -> CrosswordIterator<'_> {
         CrosswordIterator {
             crossword: self,
             position: 0,
         }
     }
 
-    pub fn cell(&self, x: usize, y: usize) -> CrosswordCell {
+    pub fn cell(&self, x: usize, y: usize) -> CrosswordCell<'_> {
         CrosswordCell {
             x,
             y,