Made everything a bit more rustic

src/common/file.rs

@@ -5,13 +5,16 @@ fn format_path(day_num: usize, file: &str) -> String {
     format!("data/day{day_num:02}/{file}")
 }
 
-pub fn read_lines(day_num: usize, file: &str) -> io::Result<Vec<String>> {
-    Ok(fs::read_to_string(format_path(day_num, file))?
+pub fn read_string(day_num: usize, file: &str) -> io::Result<String> {
+    Ok(fs::read_to_string(format_path(day_num, file))?)
+}
+
+pub fn split_lines<'a>(lines: &'a str) -> impl Iterator<Item = &'a str> + 'a {
+    lines
         .split('\n')
         .with_position()
         .filter_map(|(pos, line)| match pos {
             itertools::Position::Last if line.is_empty() => None,
-            _ => Some(line.to_owned()),
+            _ => Some(line),
         })
-        .collect())
 }