WIP: replace repl_show with display

Working on refactoring printing code to use `display` instead of `repl_show`, and get rid of some of the too-many printing functions.

`repl_show` becomes `display` when being called, and `writemime` when adding definitions, which makes it tricky to deprecate.

This will affect a few packages. We also have to decide how this relates to Winston's and ImageView's `display` function. I think the best option is to have those packages push Displays that open windows and such, and have the repl call 'display'.