Bugs / design issues:
- Fix cursor cropping in render function where missing
- Deal with hRelease/vRelease differently, e.g. by using Maybe to
contain width and height in the context, and behave differently if
they're Nothing (same for boxes?)
- Do we need CropLeftBy and CropTopBy if we just Translate negatively?

Features:
- Text manipulation API
  - monoid instance
  - RLE attribute assignment with successive transformations?
  - wrapping
- Think of better library name
- Deal with multi-column characters everywhere
- Port widgets from vty-ui

Performance:
- Write benchmarks for render on various primitive types
- Use a zipper for the list widget, use vectors for storage
- Use Vector instead of List where sensible