Testing out a line-breaking type

Just a quick project that I'm planning to use in bigger things. The main difference is that it assumes ASCII but not UTF-8. Want people that are not me to make sure I didn't miss anything.

Hmm, it can use Sequence and Collection inputs, but is there any gotchas if I complete the set with AsyncSequence sources? What are good ways to test the Task cancelled/ended code, including when the task ended before the first element is produced (if that means anything)?