325: implement translate_offset_with_edit r=matklad a=vemoo
- Implement `translate_offset_with_edit` to resolve#105
- Add proptest impls for text, offsets and edits and use them in tests for `translate_offset_with_edit` and `LineIndex`
- Added benchmark for `translate_offset_with_edit`
Co-authored-by: Bernardo <berublan@gmail.com>