Refactor initial chunking to be line‐based

Refactor initial chunking to be line‐based

The old parsing mechanism operated primarily on large string chunks,
which were re‐parsed into lines potentially multiple times. This
refactor changes the parsing to use lines and ranges, which is a little
more verbose/complicated from an X·Path perspective (it requires a lot
of `generate-id()` comparisons) but hopefully, on the whole, better.