- Use lenses in core library
- Fix cursor cropping in render function where missing
- Deal with multi-column characters everywhere
- Implement 2-d text editing
- Write benchmarks for render on various primitive types
- Use Vector instead of List where sensible
- Think about porting some widgets from vty-ui
- Think of better library name
- Support "suggested" vs "forced" attributes in Prim rendering
- List: add listGetSelected :: List a -> Maybe (Int, a)
- List: support a cursorless mode, where scrolling always scrolls up and
  down as if the cursor was already at the top or bottom of the window
- Do we need CropLeftBy and CropTopBy if we just Translate negatively?
- 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?)

Bugs:
- List element height maintenance breaks when we insert into the middle
  of the list