When users resize the terminal, they expect lines that we're automatically wrapped to re-wrap to the new size. A simple flag on each line to track whether it was wrapped should suffice. Resize logic can come later

Add a way to resize a termrect, re-wrapping as necessary (see #4)

Tendril is a nice zero-copy string slice type from the servo project. Since some workloads of termrect will end up doing a lot of string slicing, copying strings around might end up being a little bit slower. Tendril might make things faster.

There should be some kind of scrollable screenrect that can keep many more lines than what fits on the screen. It might not be able to wrap a screenrect. It actually might be better here to make the regular screenrect able to hold more lines and just limit with a setting of some kind.