DeprecatedString::substring() makes a copy of the substring.
Instead, use a StringView, which can make substring views in constant
time.
Reduces time for `pdf --dump-contents image-based-pdf-sample.pdf` to
2.2s (from not completing for 1+ minutes).
That file contains a 221 kB jpeg.
Find it on the internet here:
https://nlsblog.org/wp-content/uploads/2020/06/image-based-pdf-sample.pdf
93b3f12680