Add iterators

[mcy]

This adds something like Rust's Iterator trait but in a way that is suitable for C++ use. We have:

• best::iter and friends.
• best::int_range, which best::bounds is now a specialization of. It's an iterator.
• best::span::find() and best::text::find() now have standardized interfaces.
• best::span::split(), best::text::split().
• best::text::runes() and best::text::rune_indices().
• best::pretext, i.e., a best::text that has not been validated.

Removed best::rune::iter, since it was a stopgap solution until we got here.