Use character offsets in diff.

src/compare/diff.rs

@@ -115,12 +115,13 @@ impl<'s> DiffResult<'s> {
             }
         };
         let rust_offset = self.rust_source.as_ptr() as usize - original_document.as_ptr() as usize;
+        let preceding_text = &original_document[..rust_offset];
         println!(
             "{indentation}{status_text} {name} char({char_offset}) {message}",
             indentation = "  ".repeat(indentation),
             status_text = status_text,
             name = self.name,
-            char_offset = rust_offset + 1,
+            char_offset = preceding_text.chars().count() + 1,
             message = self.message.as_ref().map(|m| m.as_str()).unwrap_or("")
         );
         for child in self.children.iter() {