Fix clippy errors.

src/compare/compare_field.rs

@@ -655,7 +655,7 @@ pub(crate) fn compare_property_number_lines<
         (Some(number_lines), Some(rust_number_lines)) => {
             let token_list = number_lines.as_list()?;
             let number_type = token_list
-                .get(0)
+                .first()
                 .map(Token::as_atom)
                 .map_or(Ok(None), |r| r.map(Some))?
                 .ok_or(":number-lines should have a type.")?;

src/compare/diff.rs

@@ -2153,7 +2153,7 @@ fn compare_plain_text<'b, 's>(
     let text = emacs.as_text()?;
     let start_ind: usize = text
         .properties
-        .get(0)
+        .first()
         .expect("Should have start index.")
         .as_atom()?
         .parse()?;

src/parser/citation.rs

@@ -235,7 +235,7 @@ mod tests {
         assert_eq!(
             first_paragraph
                 .children
-                .get(0)
+                .first()
                 .expect("Len already asserted to be 1"),
             &Object::Citation(Citation {
                 source: "[cite:@foo]",