Support regex split #44

split takes a literal, non-empty separator because decode joins the parts back with that same separator. With a pattern, nothing records which text matched, so the join is not determined.

Decision: decline. Besides the undetermined join, a regular expression in a crosswalk is evaluated by a synchronous executor over document data, and JavaScript's engine backtracks: a crafted pattern can take super-linear work on a short input. Crosswalks are data the codec validates, and the command reads them from URLs (#48), so a pattern field would let a crosswalk author make every encode call arbitrarily expensive. Recording the matched separator per boundary would keep the round trip but not remove that cost.

Revisit if a real source uses a small set of literal separators: a split over a declared list of literal separators that records which one occurred at each boundary is invertible and runs in linear time.

The README reason for this omission is updated in #56.