linewise operations paying attention to "screen lines"

Trying to treat folds as single lines for the linewise operations.

There was some discussion of this in chat and also in person. It may end up with a third set of coördinates, but for now I'm just trying to go for screen as opposed to buffer. And I think I may be doing this entirely incorrectly.

Please feel free to point me in the right direction, @atom/core, @mcolyer.