properly use safe-string's lines() method now that is available

src/span.rs

@@ -129,20 +129,17 @@ impl Span {
     }
 
     /// Returns an iterator over the lines of the [`Source`] that this [`Span`] is associated with,
-    pub fn source_lines(&self) -> impl Iterator<Item = (IndexedString, Range<usize>)> + '_ {
-        // TODO: re-write entirely using `IndexedString` and `IndexedSlice`
+    pub fn source_lines(&self) -> impl Iterator<Item = (IndexedSlice, Range<usize>)> + '_ {
         let start_line_col = self.start();
         let end_line_col = self.end();
         let start_col = start_line_col.col;
         let start_line = start_line_col.line;
         let end_line = end_line_col.line;
         let end_col = end_line_col.col;
         self.source
-            .as_str()
             .lines()
             .enumerate()
             .filter_map(move |(i, line)| {
-                let line = IndexedString::from(line);
                 let len = line.len();
                 if start_line == end_line && end_line == i {
                     Some((line, start_col..end_col))