audit uses of `tibble()` in internal functions

[simonpcouch]

          I have no arguments here, but I'm curious why you made that change. Is it for readability (since these aren't likely to be called a large number of times)?

Originally posted by @topepo in #51 (comment)

tunable.equivocal_zone() can probably use tibble::new_tibble(list())--just needs some ad-hoc recycling.