fix: use `str::char_indices` for `Deroffer::str_at`

[scooter-dangle]

It has become more awkward for str_at to be unable to handle strings with multi-byte characters.

We might still return later and convert many of these functions to operate on byte-strings rather than strings, in which case we'll want to revert this change.

Note: Motivated by #46 and #47.