Use byte count for token span in the lexer.

Somehow, miette doesn't play well with spans when using chars indices.
  So we have to count the number of bytes in strings / chars, so that
  spans align accordingly.

crates/aiken-lang/src/parser/extra.rs

@@ -23,16 +23,12 @@ pub struct Comment<'a> {
 
 impl<'a> From<(&Span, &'a str)> for Comment<'a> {
     fn from(src: (&Span, &'a str)) -> Comment<'a> {
-        fn char_indice(s: &str, i: usize) -> usize {
-            s.char_indices().nth(i).unwrap_or((i, ' ')).0
-        }
-
-        let start = char_indice(src.1, src.0.start);
-        let end = char_indice(src.1, src.0.end);
-
+        let start = src.0.start;
+        let end = src.0.end;
         Comment {
-            start: src.0.start,
-            content: src.1.get(start..end).expect("From span to comment"),
+            start,
+            content: std::str::from_utf8(src.1.as_bytes()[start..end].as_ref())
+                .expect("From span to comment"),
         }
     }
 }